gated.conf(SFF)

Description

Statement grouping

1. options

2. interfaces

3. definition statements: autonomoussystem, routerid, and martians

4. protocol statements: rip, ospf, egp, bgp, icmp, redirect, routerdiscovery, snmp, and kernel

5. static

6. control statements: import, and export

7. aggregate

Two other types of statements do not fit in these categories: %directive and %trace. These statements provide instructions to the parser and control tracing from the configuration file. They do not relate to the configuration of any protocol and may occur anywhere in the gated.conf file.

Statement summary

Command	Type	Description
%directory	directive	Sets the directory for include files
%include	directive	Includes a file into gated.conf
traceoptions	trace	Specifies which events are traced
options	definition	Defines gated options
interfaces	definition	Defines gated interfaces
autonomoussystem	definition	Defines the AS number
routerid	definition	Defines the originating router (BGP, OSPF)
martians	definition	Defines invalid destination addresses
rip	protocol	Enables the RIP protocol
kernel	protocol	Configures kernel interface options
ospf	protocol	Enables the OSPF protocol
egp	protocol	Enables the EGP protocol
bgp	protocol	Enables the BGP protocol
redirect	protocol	Configures the processing of ICMP redirects
icmp	protocol	Configures the processing of general ICMP packets
snmp	protocol	Enables reporting to SNMP
static	static	Defines static routes
import	control	Defines which routes to import
export	control	Defines which routes to export
aggregate	control	Defines which routes to aggregate
generate	control	Defines which routes to generate

 +-----------------+------------+-----------------------------------+
 |Command          | Type       | Description                       |
 +-----------------+------------+-----------------------------------+
 |%directory       | directive  | Sets the directory for include    |
 |                 |            | files                             |
 +-----------------+------------+-----------------------------------+
 |%include         | directive  | Includes a file into gated.conf   |
 +-----------------+------------+-----------------------------------+
 |traceoptions     | trace      | Specifies which events are traced |
 +-----------------+------------+-----------------------------------+
 |options          | definition | Defines gated options             |
 +-----------------+------------+-----------------------------------+
 |interfaces       | definition | Defines gated interfaces          |
 +-----------------+------------+-----------------------------------+
 |autonomoussystem | definition | Defines the AS number             |
 +-----------------+------------+-----------------------------------+
 |routerid         | definition | Defines the originating router    |
 |                 |            | (BGP, OSPF)                       |
 +-----------------+------------+-----------------------------------+
 |martians         | definition | Defines invalid destination       |
 |                 |            | addresses                         |
 +-----------------+------------+-----------------------------------+
 |rip              | protocol   | Enables the RIP protocol          |
 +-----------------+------------+-----------------------------------+
 |kernel           | protocol   | Configures kernel interface       |
 |                 |            | options                           |
 +-----------------+------------+-----------------------------------+
 |ospf             | protocol   | Enables the OSPF protocol         |
 +-----------------+------------+-----------------------------------+
 |egp              | protocol   | Enables the EGP protocol          |
 +-----------------+------------+-----------------------------------+
 |bgp              | protocol   | Enables the BGP protocol          |
 +-----------------+------------+-----------------------------------+
 |redirect         | protocol   | Configures the processing of ICMP |
 |                 |            | redirects                         |
 +-----------------+------------+-----------------------------------+
 |icmp             | protocol   | Configures the processing of      |
 |                 |            | general ICMP packets              |
 +-----------------+------------+-----------------------------------+
 |snmp             | protocol   | Enables reporting to SNMP         |
 +-----------------+------------+-----------------------------------+
 |static           | static     | Defines static routes             |
 +-----------------+------------+-----------------------------------+
 |import           | control    | Defines which routes to import    |
 +-----------------+------------+-----------------------------------+
 |export           | control    | Defines which routes to export    |
 +-----------------+------------+-----------------------------------+
 |aggregate        | control    | Defines which routes to aggregate |
 +-----------------+------------+-----------------------------------+
 |generate         | control    | Defines which routes to generate  |
 +-----------------+------------+-----------------------------------+

Preference

Preference can be set based on network interface over another, from one protocol over another, or from one remote gateway over another.

Preference may not be used to control the selection of routes within an igp, this is accomplished automatically by the protocol based on metric. Preference may be used to select routes from the same egp learned from different peers or autonomous systems.

Each route has only one preference value associated with it, even though preference can be set at many places in the configuration file. Simply, the last or most specific preference value set for a route is the value used. The preference value is an arbitrarily assigned value used to determine the order of routes to the same destination in a single routing database. The active route is chosen by the lowest preference value.

Some protocols implement a second preference (preference2), sometimes referred to as a ``tie-breaker''. The following algorithm is used to select a route:

1. The route with the best (numerically smallest) preference is preferred.

2. If the two routes have the same preference, the route with the best (numerically smallest) preference2 (also known as a tie-breaker) is preferred.

3. A route learned from a igp is preferred to a route learned from an egp. Least preferred is a route learned indirectly by an igp from an egp.

4. If AS path information is available it is used to help determine the most preferred route:

   
   4a.

   A route with an AS path is preferred over one without an AS path.

   
   4b.

   If the AS paths and origins are identical, the route with the lower metric is preferred.

   
   4c.

   A route with an AS path origin of igp is preferred over a route with an AS path origin of egp. Least preferred is an AS path with an unknown origin.

   
   4d.

   A route with a shorter AS path is preferred.

5. If both routes are from the same protocol and AS, the one with the lowest metric is preferred.

6. The route with the lowest numeric next-hop address is used.

Assigning preferences

The following table summarizes the default preference values for routes learned in various ways. The table lists the statements (some of these are clauses within statements) that set preference, and shows the types of routes to which each statement applies. The default preference for each type of route is listed, and the table notes preference precedence between protocols. The narrower the scope of the statement, the higher precedence its preference value is given, but the smaller the set of routes it affects.

Preference of	Defined by statement	Default
Directly connected networks	interface	0
OSPF routes	ospf	10
Internally generated default	gendefault	20
Redirects	redirect	30
Routes learned via route socket	kernel	40
Static routes from config	static	60
RIP routes	rip	100
Point-to-point interface		110
Routes to interfaces that are down	interfaces	120
Aggregated/generated routes	aggregate/generate	130
OSPF AS external routes	ospf	150
BGP routes	bgp	170
EGP	egp	200

 +-----------------------------------+----------------------+---------+
 |Preference of                      | Defined by statement | Default |
 +-----------------------------------+----------------------+---------+
 |Directly connected networks        | interface            | 0       |
 +-----------------------------------+----------------------+---------+
 |OSPF routes                        | ospf                 | 10      |
 +-----------------------------------+----------------------+---------+
 |Internally generated default       | gendefault           | 20      |
 +-----------------------------------+----------------------+---------+
 |Redirects                          | redirect             | 30      |
 +-----------------------------------+----------------------+---------+
 |Routes learned via route socket    | kernel               | 40      |
 +-----------------------------------+----------------------+---------+
 |Static routes from config          | static               | 60      |
 +-----------------------------------+----------------------+---------+
 |RIP routes                         | rip                  | 100     |
 +-----------------------------------+----------------------+---------+
 |Point-to-point interface           |                      | 110     |
 +-----------------------------------+----------------------+---------+
 |Routes to interfaces that are down | interfaces           | 120     |
 +-----------------------------------+----------------------+---------+
 |Aggregated/generated routes        | aggregate/generate   | 130     |
 +-----------------------------------+----------------------+---------+
 |OSPF AS external routes            | ospf                 | 150     |
 +-----------------------------------+----------------------+---------+
 |BGP routes                         | bgp                  | 170     |
 +-----------------------------------+----------------------+---------+
 |EGP                                | egp                  | 200     |
 +-----------------------------------+----------------------+---------+

Sample preference specifications

interfaces {
	interface 138.66.12.2 preference 10 ;
} ;
rip yes {
	preference 90 ;
} ;
import proto rip gateway 138.66.12.1 preference 75 ;

In these statements the preference applicable to routes learned via RIP from gateway 138.66.12.1 is 75. The last preference applicable to routes learned via RIP from gateway 128.66.12.1 is defined in the accept statement. The preference applicable to other RIP routes is found in the rip statement. The preference set on the interface statement applies only to the route to that interface.

Trace statements

Global tracing options

The trace flags that only have global significance are:

parse

Trace the lexical analyzer and parser. Mostly used by gated developers for debugging.

adv

Trace the allocation of and freeing of policy blocks. Mostly used by the gated developers for debugging.

symbols

Used to trace symbols read from the kernel at startup. The only useful way to specify this level of tracing is via the -t option on the command line since the symbols are read from the kernel before parsing the configuration file.

iflist

Used to trace the reading of the kernel interface list. It is useful to specify this with the -t option on the command line since the first interface scan is done before reading the configuration file.

all

Turn on all of the following.

general

A shorthand notation for specifying both normal and route.

state

Trace state machine transitions in the protocols.

normal

Trace normal protocols occurrences. Abnormal protocol occurrences are always traced.

policy

Trace application of protocol and user-specified policy to routes being imported and exported.

task

Trace system interface and processing associated with this protocol or peer.

timer

Trace timer usage by this protocol or peer.

route

Trace routing table changes for routes installed by this protocol or peer.

It is not currently possible to specify packet tracing from the command line. This is because a global option for packet tracing would potentially create too much output.

When protocols inherit their tracing options from the global tracing options, tracing levels that do not make sense (such as parse, adv and packet tracing options) are masked out.

Global tracing statements have an immediate effect, especially parsing options that effect the parsing of the configuration file. Tracing values inherited by protocols specified in the configuration file are initially inherited from the global options in effect as they are parsed, unless they are overridden by more specific options. After the configuration file is read, tracing options that were not explicitly specified are inherited from the global options in effect at the end of the configuration file.

Packet tracing

detail

Normally packets are traced in a terse form of one or two lines. When detail is specified, a more verbose format is used to provide further details about the contents of the packet.

send

recv

These options limit the tracing to packets sent or received. Without these options both sent and received packets will be traced.

Trace options syntax

trace_file

Specifies the file to receive tracing information. If this file name does not begin with a slash (/) the directory where gated was started in prepended to the name.

replace

Tracing should start by replacing an existing file. The default is to append to an existing file.

size size[k|m] files files

Limit the maximum size of the trace file to the specified size (minimum 10k). When the trace file reaches the specified size, it is renamed to file.0, then file.1, file.2 up to the maximum number of files (minimum specification is 2).

control_options

Specifies options that control the appearance of tracing. Valid values are:

nostamp

Specifies that a timestamp should not be prepended to all trace lines.

except trace_options

Used to enable a broad class of tracing and then disable more specific options.

none

Specifies that all tracing should be turned off for this protocol or peer.

Directive statements

%directory directory

Defines the directory where the include files are stored. When it is used, gated looks in the directory identified by pathname for any included files that do not have a fully qualified filename that is, do not begin with ``/''. This statement does not actually change the current directory, it just specifies the prefix applied to included file names.

%include filename

Identifies an include file. The contents of the file is included in the gated.conf file at the point where the %include directive is encountered. If the filename is not fully qualified, that is, does not begin with ``/'', it is considered to be relative to the directory defined in the %directory directive. The %include directive statement causes the specified file to be parsed completely before resuming with this file. Nesting up to ten levels is supported.

Options statements

The options statement syntax is:

options
[ nosend ]
[ noresolv ]
[ gendefault [ preference preference ] [ gateway gateway] ]
[ syslog [ upto ] log_level ]
[ mark time ]
;

The options list can contain one or more of the following options:

gendefault [ preference preference ] [ gateway gateway]

When gendefault is enabled, when a BGP or EGP neighbor is up, it causes the creation of a default route with the special protocol default. This can be disabled per BGP/EGP group with the nogendefault option. By default, this route has a preference of 20. This route is normally not installed in the kernel forwarding table, it is only present so it can be announced to other protocols. If a gateway is specified, the default route will be installed in the kernel forwarding table with a next hop of the listed gateway.

Note that the use of the more general option is preferred to the use of this gendefault option. The gendefault option may go away in future releases. See ``Route aggregation'' for more information on the generate statement.

nosend

Do not send any packets. This option makes it possible to run gated on a live network to test protocol interactions without actually participating in the routing protocols. The packet traces in the gated log can be examined to verify that gated is functioning properly. This is most useful for RIP and possibly the SMUX SNMP interface. This option does not yet apply to BGP and is less than useful with EGP and OSPF.

noresolv

By default gated will try to resolve symbolic names into IP addresses by using the gethostbyname and getnetbyname library calls. These calls usually use the Domain Name System (DNS) instead of the hosts local host and network tables. If there is insufficient routing information to send DNS queries, gated will deadlock during startup. This option can be used to prevent these calls, symbolic names will result in configuration file errors.

syslog [ upto ] log_level

Controls the amount of data gated logs via syslog. The available logging levels and other terminology are as defined on the syslog(S) manual page. The default is equivalent to syslog upto info.

mark time

Specifying this option causes gated to output a message to the trace log at the specified interval. This can be used as one method of determining if gated is still running.

Interfaces statements

An interface is the connection between a router and one of its attached networks. A physical interface may be specified by interface name, by IP address, or by domain name, (unless the network is an unnumbered point-to-point network.) Multiple levels of reference in the configuration language allow identification of interfaces using wildcard, interface type name, or delete word address. Be careful with the use of interface names as it is possible to have more than one address per interface. The interface_list is a list of one or more interface names including wildcard names (names without a number) and names which may specify more than one interface or address, or the token all for all interfaces.

options

Allows configuration of some global options related to interfaces. These are:

strictinterfaces

Indicates that it is a fatal error to reference an interface in the configuration file that is not present when gated is started and not listed in a define statement. Without this option a warning message will be issued but gated will continue.

scaninterval time

Specifies how often gated scans the kernel interface list for changes. The default is every 15 seconds. Note that gated will also scan the interface list on receipt of a SIGUSR2.

interface interface_list

Sets interface options on the specified interfaces. An interface list is all or a list of interface names (see warning about interface names), domain names, or numeric addresses. Options available on this statement are:

preference preference

Sets the preference for routes to this interface when it is up and appears to be functioning properly. The default preference is 0.

down preference preference

Sets the preference for routes to this interface when gated does not believe it to be functioning properly, but the kernel does not indicate it is down. The default value is 120.

passive

Prevents gated from changing the preference of the route to this interface if it is not believed to be functioning properly due to lack of received routing information. gated will only perform this check if the interface is actively participating in a routing protocol.

simplex

Defines an interface as unable to hear its own broadcast packets. Some systems define an interface as simplex with the IFF_SIMPLEX flag, on others it needs to be specified in the configuration file. On simplex interfaces, packets from myself are assumed to have been looped back in software and are not used as an indication that the interface is functioning properly.

define address

Defines interfaces that might not be present when gated is started so they may be referenced in the configuration file when strictinterfaces is defined. Possible define keywords are:

broadcast address

Defines the interface as broadcast capable (for example, Ethernet or Token Ring) and specifies the broadcast address.

pointopoint address

Defines the interface as a pointopoint interface (for example, SLIP or PPP) and specifies the address on the local side. The first address on the define statement references the address of the host on the remote end of the interface, the address specified after this pointopoint keyword defines the address on the local side of the interface.

netmask mask

Specifies the subnetmask to be used on this interface. This is ignored on pointtopoint interfaces.

multicast

Specifies that the interface is multicast capable.

Interface list specification

all

This refers to all available interfaces.

Interface name wildcard

This refers to all the interfaces of the same type. DLPI and ODI interface names consist of the name of the device driver, such as ie or TCM5X9_, and a unit number such as 0 or 1. MDI interface names consist of the token net, and a unit number such as 0 or 1. Reference to the name contain only alphabetic characters and match any interfaces that have the same alphabetic part.

For example, ie would match ie0 or ie1, but it would not match iel0.

Interface name

This refers to a specific interface, usually one physical interface. These are specified as an alphabetic part followed by a numeric part. This will match one specific interface. But be aware that on many systems, there can be more than one protocol (that is, IP) address on a given physical interface. For example, ef1 will match an interface named ef1, but not an interface named ef10.

Interface address

This matches one specific interface. The reference can be by protocol address (that is, 10.0.0.51), or by symbolic host name (that is, nic.ddn.mil). Note that a symbolic host name reference is only valid when it resolves to only one address. Use of symbolic host names is not recommended.

For example, consider a system with three interfaces, le0, le1 and du0.

rip yes {
	interface all noripin noripout ;
	interface le ripin ;
	interface le1 ripout ;
} ;

IP interface addresses and routes

loopback

This interface must have the address of 127.0.0.1. Packets sent to this interface are sent back to the originator. This interface is also used as a catch all interface for implementing other features, such as reject and blackhole routes. Although a netmask is reported on this interface, it is ignored. It is useful to assign an additional address to this interface that is the same as the OSPF or BGP router id; this allows routing to a system based on the router id which will work if some interfaces are down.

broadcast

This is a multi-access interface capable of a physical level broadcast, such as Ethernet, Token Ring and FDDI. This interface has an associated subnet mask and broadcast address. The interface route to an broadcast network will be a route to the complete subnet.

point-to-point

This is a tunnel to another host, usually on some sort of serial link. This interface has a local address, and a remote address. Although it may be possible to specify multiple addresses for a point-to-point interface, there does not seem to be a useful reason for doing so.

The remote address must be unique among all the interface addresses on a given router. The local address may be shared among many point-to-point and up to one non-point-to-point interface. This is technically a form of the router id method for addressless links. This technique conserves subnets as none are required when using this technique.

If a subnet mask is specified on a point-to-point interface, it is only used by RIP version 1 to determine which subnets may be propagated to the router on the other side of this interface.

non-broadcast multi-access or nbma

This type of interface is multi-access, but not capable of broadcast.

For point-to-point interfaces, gated installs some special routes. If the local address on one or more point-to-point interfaces is not shared with a non-point-to-point interface, gated installs a route to the local address pointing at the loopback interface with a preference of 110. This insures that packets originating on this host destined for this local address are handled locally. OSPF prefers to route packets for the local interface across the point-to-point link where they will be returned by the router on the remote end. This is used to verify operation of the link. Since OSPF installs routes with a preference of 10, these routes will override the route installed with a preference of 110.

If the local address of one or more point-to-point interfaces is shared with a non-point-to-point interface, gated installs a route to the local with a preference of 0 that will not be installed in the forwarding table. This is to prevent protocols like OSPF from routing packets to this address across a serial interface when this system could be functioning as a host.

When the status of an interface changes, gated notifies all the protocols, which take the appropriate action. gated assumes that interfaces which are not marked UP do not exist. While this might not be the most correct action, it is the way things currently work.

gated ignores any interfaces that have invalid data for the local, remote or broadcast addresses or the subnet mask. Invalid data includes zeros in any field. gated will also ignore any point-to-point interface that has the same local and remote addresses, it assumes it is in some sort of loopback test mode.

Definition statements

Autonomous system configuration

This option is required if BGP or EGP are in use. The AS number is assigned by the Network Information Center (NIC).

loops is only for protocols supporting AS paths, such as BGP. It controls the number of times this autonomous system may appear in an AS path and defaults to 1 (one).

Router ID configuration

The default is the address of the first interface encountered by gated. The address of a non-point-to-point interface is preferred over the local address of a point-to-point interface and an address on a loopback interface that is not the loopback address (127.0.0.1) is most preferred.

Martian configuration

Defines a list of martian addresses about which all routing information is ignored. Sometimes a misconfigured system sends out obviously invalid destination addresses. These invalid addresses, called martians, are rejected by the routing software. This command allows additions to the list of martian addresses. See ``Route filtering'' for more information on specifying ranges.

The allow parameter may be specified to explicitly allow a subset of a range that was disallowed.

Example definition statements

options gendefault ;
autonomoussystem 249 ;
interface 128.66.12.2 passive ;
martians {
	0.0.0.26
};

The statements in this example perform the following functions:

• The options statement tells the system to generate a default route when it peers with an EGP or BGP neighbor.

• The autonomoussystem statement tells gated to use AS number 249 for in EGP and BGP.

• The interface statement tells gated not to mark interface 128.66.12.2 as down even if it sees no traffic.

• The martians statement prevents routes to 0.0.0.26 from ever being accepted.

Protocol overview

Interior routing protocols

RIP

The Routing Information Protocol, Version 1 and Version 2, is the most commonly used interior protocol. RIP selects the route with the lowest metric as the best route. The metric is a hop count representing the number of gateways through which data must pass to reach its destination. The longest path that RIP accepts is 15 hops. If the metric is greater than 15, a destination is considered unreachable and gated discards the route. RIP assumes the best route is the one that uses the fewest gateways, that is, the shortest path, not taking into account congestion or delay on route.

The RIP version 1 protocol is described in RFC 1058 and the RIP version 2 protocol is described in RFC 1388.

OSPF

Open Shortest Path First is a link-state protocol. OSPF is better suited than RIP for complex networks with many routers. OSPF provides equal cost multipath routing.

OSPF is described in RFC 1583, and the MIB is defined in RFC 1253. Other related documents are RFC 1245, RFC 1246, and RFC 1370.

Exterior routing protocols

EGP

Exterior Gateway Protocol: Originally EGP reachability information was passed into ARPANET/MILNET ``core'' gateways where the best routes were chosen and passed back out to all connected autonomous systems. As the Internet moved toward a less hierarchical architecture, EGP, an exterior routing protocol which assumes a hierarchical structure, became less effective.

The EGP protocol is described in RFC 827 and RFC 904.

BGP

Border Gateway Protocol is replacing EGP as the exterior protocol of choice. BGP exchanges reachability information between autonomous systems, but provides more capabilities than EGP. BGP uses path attributes to provide more information about each route as an aid in selecting the best route. Path attributes may include, for example, administrative preferences based on political, organizational, or security (policy) considerations in the routing decision. BGP supports nonhierarchical topologies and can be used to implement a network structure of equivalent autonomous systems.

BGP version 1 is described in RFC 1105, version 2 in RFC 1163, version 3 in RFC 1267. The version 3 MIB is described in RFC 1269. The two documents, RFC 1164 and RFC 1268 describe the application of version 2 and three in the internet. A protocol analysis of and experience with BGP version 3 are available in RFC 1265 and RFC 1266. RFC 1397 talks about advertising a default route in BGP version 2 and 3. And finally, RFC 1403 describes BGP/OSPF interaction.

Other routing protocols

Router Discovery

The Router Discovery protocol is used to inform hosts of the availability of hosts it can send packets to and is used to supplement a statically configured default router. This is the preferred protocol for hosts to run, they are discouraged from wiretapping routing protocols.

Router Discovery is described in RFC 1256.

Routing Information Protocol (RIP)

A router running RIP in active mode broadcasts updates at set intervals. Each update contains paired values where each pair consists of an IP network address and an integer distance to that network. RIP uses a hop count metric to measure the distance to a destination. In the RIP metric, a router advertises directly connected networks at a metric of 1. Networks which are reachable through one other gateway are two hops, and so on. Thus, the number of hops or hop count along a path from a given source to a given destination refers to the number of gateways that a datagram would encounter along that path. Using hop counts to calculate shortest paths does not always produce optimal results. For example, a path with hop count 3 that crosses three Ethernets may be substantially faster that a path with a hop count 2 that crosses two slow-speed serial lines. To compensate for differences in technology many routers advertise artificially high hop counts for slow links.

A RIP routing daemon (such as route(ADMN)) dynamically builds on information received through RIP updates. When started up, it issues a REQUEST for routing information and then listens for responses to the request. If a system configured to supply RIP hears the request, it responds with a RESPONSE packet based on information in its routing database. The RESPONSE packet contains destination network addresses and the routing metric for each destination.

When a RIP RESPONSE packet is received, the routing daemon takes the information and rebuilds the routing database adding new routes and ``better'' (lower metric) routes to destinations already listed in the database. RIP also deletes routes from the database if the next router to that destination says the route contains more than 15 hops, or if the route is deleted. All routes through a gateway are deleted if no updates are received from that gateway for a specified time period. In general, routing updates are issued every 30 seconds. In many implementations, if a gateway is not heard from for 180 seconds, all routes from that gateway are deleted from the routing database. This 180 second interval also applies to deletion of specific routes.

RIP version 2

Some of the most notable RIPv2 enhancements are:

• Next hop.

  The ability to advertise a next hop to use other than the router supplying the routing update. This is quite useful when advertising a static route to a dumb router that does not run RIP as it avoids having packets destined through the dumb router from having to cross a network twice.

  RIPv1 routers will ignore next hop information in RIPv2 packets. This may result in packets crossing a network twice, which is exactly what happens with RIPv1. So this information is provided in RIPv1 compatible RIPv2 packets.

• Network mask.

  RIPv1 assumes that all subnetworks of a given network have the same network mask. It uses this assumption to calculate the network masks for all routes received. This assumption prevents subnets with different netmasks from being included in RIP packets. RIPv2 adds the ability to explicitly specify the network mask with each network in a packet.

  While RIPv1 routers will ignore the network mask in RIPv2 packets, their calculation of the network mask will quite possibly be wrong. For this reason, RIPv1-compatible RIPv2 packets must not contain networks that would be mis-interpreted. These network must only be provided in natve RIPv2 packets that are multicast.

• Authentication.

  RIPv2 packets may also contain one of two types of authentication string that may be used to verify the validity of the supplied routing data. Authentication may be used in RIPv1-compatible RIPv2 packets, but be aware that RIPv1 routers will ignore it.

  The first method is a simple password in which an authentication key of up to 16 characters is included in the packet. If this does not match what is expected, the packet will be discarded. This method provides very little security as it is possible to learn the authentication key by watching RIP packets.

  The second method is still experimental and may change in incompatible ways in future releases. This method uses the MD5 algorithm (see RFC 1321) to create a crypto-checksum of a RIP packet and an authentication key of up to 16 characters. The transmitted packet does not contain the authentication key itself, instead it contains a crypto-checksum, called the digest. The receiving router will perform a calculation using the correct authentication key and discard the packet if the digest does not match. In addition, a sequence number is maintained to prevent the replay of older packets. This method provides a much stronger assurance that routing data originated from a router with a valid authentication key.

  Two authentication methods can be specified per interface. Packets are always sent using the primary method, but received packets are checked with both the primary and secondary methods before being discarded. In addition, a separate authentication key is used for non-router queries.

RIP version 1 and network masks

On point-to-point interfaces, the netmask is applied to the remote address. The netmask on these interfaces is ignored if it matches the natural network of the remote address or if it is all ones.

Unlike in previous releases, the zero subnet mask (a network that matches the natural network of the interface, but has a more specific, or longer, network mask) is ignored. If this is not desirable, a route filter may be used to reject it.

The RIP statement

The rip statement enables or disables RIP. If the rip statement is not specified the default is rip on ;. If enabled, RIP will assume nobroadcast when there is only one interface and broadcast when there is more than one.

The options are as follows:

broadcast

Specifies that RIP packets will be broadcast regardless of the number of interfaces present. This is useful when propagating static routes or routes learned from anther protocol into RIP. In some cases, the use of broadcast when only one network interface is present can cause data packets to traverse a single network twice.

nobroadcast

Specifies that RIP packets will not be broadcast on attached interfaces, even if there are more than one. If a sourcegateways clause is present, routes will still be unicast directly to that gateway.

nocheckzero

Specifies that RIP should not make sure that reserved fields in incoming version 1 RIP packets are zero. Normally RIP will reject packets where the reserved fields are zero.

preference preference

Sets the preference for routes learned from RIP. The default preference is 100. This preference may be overridden by a preference specified in import policy.

defaultmetric metric

Defines the metric used when advertising routes via RIP that were learned from other protocols. If not specified, the default value is 16 (unreachable). This choice of values requires you to explicitly specify a metric in order to export routes from other protocols into RIP. This metric may be overridden by a metric specified in export policy.

query authentication [none | [[simple|md5] password]] ;

Specifies the authentication required of query packets that do not originate from routers. The default is none.

interface interface_list

Controls various attributes of sending RIP on specific interfaces. See ``Interface list specification'' for the description of the interface_list.

Note that if there are multiple interfaces configured on the same subnet, RIP updates will only be sent from the first one for which RIP output is configured. This limitation is required because of the way the kernel operates. It will hopefully be removed in a future release.

The possible parameters are:

noripin

Specifies that RIP packets received via the specified interface will be ignored. The default is to listen to RIP packets on all non-loopback interfaces.

ripin

This is the default. This argument may be necessary when noripin is used on a wildcard interface descriptor.

noripout

Specifies that no RIP packets will be sent on the specified interfaces. The default is to send RIP on all broadcast and non-broadcast interfaces when in broadcast mode. The sending of RIP on point-to-point interfaces must be manually configured.

ripout

This is the default. This argument is necessary when sending RIP on point-to-point interfaces and may be necessary when noripin is used on a wildcard interface descriptor.

metricin metric

Specifies the RIP metric to add to incoming routes before they are installed in the routing table. The default is the kernel interface metric plus 1 (which is the default RIP hop count). If this value is specified it will be used as the absolute value, the kernel metric will not be added. This option is used to make this router prefer RIP routes learned via the specified interface(s) less than RIP routes from other interfaces.

metricout metric

Specifies the RIP metric to be added to routes that are send via the specified interface(s). The default is zero. This option is used to make other routers prefer other sources of RIP routes over this router.

version 1

Specifies that RIP packets send via the specified interface(s) will be version 1 packets. This is the default.

version 2

Specifies that RIP version 2 packets will be sent on the specified interfaces(s). If IP multicast support is available on this interface the default is to send full version 2 packets. If it is not available, version 1 compatible version 2 packets will be sent.

multicast

Specifies that RIP version 2 packets should be multicast on this interface. This is the default.

broadcast

Specifies that RIP version 1 compatible version 2 packets should be broadcast on this interface, even if IP multicast is available.

[secondary] authentication [none | [simple|md5] password]

This defines the authentication type to use. It applies only to RIP version 2 and is ignored for RIP-1 packets. The default authentication type is none. If a password is specified, the authentication type defaults to simple. The password should be a quoted string with between 0 and 16 characters.

If secondary is specified, this defines the secondary authentication. If omitted, the primary authentication is specified. The default is primary authentication of none and no secondary authentication.

trustedgateways gateway_list

Defines the list of gateways from which RIP will accept updates. The gateway_list is simply a list of host names or IP addresses. By default, all routers on the shared network are trusted to supply routing information. But if the trustedgateways clause is specified only updates from the gateways in the list are accepted.

sourcegateways gateway_list

Defines a list of routers to which RIP sends packets directly, not through multicast or broadcast. This can be used to send different routing information to specific gateways. Updates to gateways in this list are not affected by noripout on the interface.

traceoptions trace_options

Specifies the tracing options for RIP. (See ``Trace statements'' and ``Tracing options''.)

Tracing options

Packet tracing options (which may be modified with detail, send or recv):

packets

All RIP packets.

request

RIP information request packets, such as REQUEST, POLL and POLLENTRY.

response

RIP RESPONSE packets, which is the type of packet that actually contains routing information.

other

Any other type of packet. The only valid ones are TRACE_ON and TRACE_OFF, both of which are ignored.

The OSPF protocol

OSPF allows networks to be grouped into areas. Routing information passed between areas is abstracted, potentially allowing a significant reduction in routing traffic. OSPF uses four different types of routes, listed in order of preference: intra-area, inter-area, type 1 external and type 2 external. Intra-area paths have destinations within the same area, inter-area paths have destinations in other OSPF areas and Autonomous System External (ASE) routes are routes to destinations external to the AS. Routes imported into OSPF as type 1 routes are supposed to be from igps whose external metrics are directly comparable to OSPF metrics. When a routing decision is being made, OSPF will add the internal cost to the AS Border router to the external metric. Type 2 ASEs are used for egps whose metrics are not comparable to OSPF metrics. In this case, only the internal OSPF cost to the AS Border router is used in the routing decision.

From the topology database, each router constructs a tree of the shortest paths with itself as the root. This shortest-path tree gives the route to each destination in the AS. Externally derived routing information appears on the tree as leaves. The link-state advertisement format distinguishes between information acquired from external sources and information acquired from internal routers, so there is no ambiguity about the source or reliability of routes. Externally derived routing information (for example, routes learned from EGP or BGP) is passed transparently through the autonomous system and is kept separate from OSPF's internally derived data. Each external route can also be tagged by the advertising router, enabling a passing of additional information between routers on the borders of the autonomous system.

OSPF optionally includes type of service (TOS) routing and allows administrators to install multiple routes to a given destination for each type of service (for example, low delay or high throughput.) A router running OSPF uses the destination address and the type of service to choose the best route to the destination.

OSPF intra- and inter-area routes are always imported into the gated routing database with a preference of 10. It would be a violation of the protocol if an OSPF router did not participate fully in the area's OSPF, so it is not possible to override this. Although it is possible to give other routes lower preference values explicitly, it is ill-advised to do so.

Hardware multicast capabilities are also used where possible to deliver link-status messages. OSPF areas are connected by the backbone area, the area with identifier 0.0.0.0. All areas must be logically contiguous and the backbone is no exception. To permit maximum flexibility, OSPF allows the configuration of virtual links. These allow the backbone area to appear contiguous despite the physical reality.

All routers in an area must agree on that area's parameters. A separate copy of the link-state algorithm is run for each area. Because of this, most configuration parameters are defined on a per area basis. All routers belonging to an area must agree on that area's configuration. Misconfiguration will lead to adjacencies not forming between neighbors, and routing information might not flow, or even loop.

Authentication

The simple password provides very little protection because in many cases it is possible to easily capture packets from the network and learn the authentication key. The experimental MD5 algorithm provides much more protection as it does not include the authentication key in the packet.

The OSPF specification currently specifies that the authentication type be configured per area with the ability to configure separate passwords per interface. This has been extended to allow the configuration of different authentication types and keys per interface. In addition it is possible to specify both a primary and a secondary authentication type and key on each interface. Outgoing packets use the primary authentication type, but incoming packets may match either the primary or secondary authentication type and key.

The OSPF statement

The following are the interface_parameters referred to above. The may be specified on any class of interface and are described under the interface clause.

enable | disable ;
retransmitinterval time ;
transitdelay time ;
priority priority ;
hellointerval time ;
routerdeadinterval time ;
authkey auth_key ;

The parameters to defaults specify the defaults used when importing OSPF ASE routes into the gated routing table and exporting routes from the gated routing table into OSPF ASEs.

preference preference

The preference is used to determine how OSPF routes compete with routes from other protocols in the gated routing table. The default value is 150.

cost cost

The cost is used when exporting a non-OSPF route from the gated routing table into OSPF as an ASE. The default value is 1. This may be explicitly overridden in export policy.

tag [ as ] tag

OSPF ASE routes have a 32-bit tag field that is not used by the OSPF protocol, but may be used by export policy to filter routes. When OSPF is interacting with an egp, the tag field may be used to propagate AS path information, in which case the as keyword is specified and the tag is limited to 12 bits of information. If not specified, the tag is set to zero.

type 1 | 2

Routes exported from the gated routing table into OSPF default to becoming type 1 ASEs. This default may be explicitly changed here and overridden in export policy.

exportinterval time

This specifies how often a batch of ASE link state advertisements will be generated and flooded into OSPF. The default is once per second.

exportlimit routes

This parameter specifies how many ASEs will be generated and flooded in each batch. The default is 100.

traceoptions trace_options

Specifies the tracing options for OSPF. (See ``Trace statements'' and ``OSPF tracing options''.)

monitorauthkey authkey

OSPF state may be queried using the ospf_monitor (this should be a hyperlink) utility. This utility sends non-standard OSPF packets which generate a text response from OSPF. By default these requests are not authenticated, if an authentication key is configured, the incoming requests must match the specified authentication key. No OSPF state may be changed by these packets, but the act of querying OSPF can utilize system resources.

backbone

area area

Each OSPF router must be configured into at least one OSPF area. If more than one area is configured, at least one must be the backbone. The backbone may only be configured using the backbone keyword, it may not be specified as area 0. The backbone interface may be a virtuallink.

authtype 0 | 1 | none | simple

OSPF specifies an authentication scheme per area. Each interface in the area must use this same authentication scheme although it may use a different authenticationkey. The currently valid values are none (0) for no authentication, or simple (1) for simple password authentication.

stub [ cost cost]

A stub area is one in which there are no ASE routes. If a cost is specified, this is used to inject a default route into the area with the specified cost.

networks

The networks list describes the scope of an area. Intra-area LSAs that fall within the specified ranges are not advertised into other areas as inter-area routes. Instead, the specified ranges are advertised as ``summary network'' LSAs. If restrict is specified, the summary network LSAs are not advertised. Intra-area LSAs that do not fall into any range are also advertised as summary network LSAs. This option is very useful on well designed networks in reducing the amount of routing information propagated between areas. The entries in this list are either networks, or a subnetwork/mask pair. See ``Route filtering'' for more details about specifying ranges.

stubhosts

This lists specifies directly attached hosts that should be advertised as reachable from this router and the costs they should be advertised with. Point-to-point interfaces on which it is not desirable to run OSPF should be specified here.

It is also useful to assign an additional address to the loopback interface (one not on the 127 network) and advertise it as a stub host. If this address is the same one used as the router-id it enables routing to OSPF routers by router-id, instead of by interface address. This is more reliable than routing to one of the routers interface addresses which may not always be reachable.

interface interface_list [cost cost ]

This form of the interface clause is used to configure a broadcast (which requires IP multicast support) or a point-to-point interface. See ``Interface list specification'' for the description of the format of interface_list.

Each interface has a cost. The costs of all interfaces a packet must cross to reach a destination are summed to get the cost to that destination. The default cost is one, but another non-zero value may be specified.

Interface parameters common to all types of interfaces are:

retransmitinterval time

The number of seconds between link state advertisement retransmissions for adjacencies belonging to this interface.

transitdelay time

The estimated number of seconds required to transmit a link state update over this interface. transitdelay takes into account transmission and propagation delays and must be greater than 0.

priority priority

A number between 0 and 255 specifying the priority for becoming the designated router on this interface. When two routers attached to a network both attempt to become designated router, the one with the highest priority wins. A router whose router priority is set to 0 is ineligible to become designated router.

hellointerval time

The length of time, in seconds, between Hello packets that the router sends on the interface.

routerdeadinterval time

The number of seconds not hearing a router's Hello packets before the router's neighbors will declare it down.

authkey auth_key

Used by OSPF authentication to generate and verify the authentication field in the OSPF header. The authentication key can be configured on a per interface basis. It is specified by one to eight decimal digits separated by periods, a one to eight-byte hexadecimal string preceded by 0x, or a one to eight character string in double quotes.

nomulticast

By default, OSPF packets to neighbors on point-to-point interfaces are sent via the IP multicast mechanism.

If the use of IP multicasting is not desired because the remote neighbor does not support it, the nomulticast parameter may be specified to force the use of unicast OSPF packets.

interface interface_list nonbroadcast [cost cost ]

This form of the interface clause is used to specify a nonbroadcast interface on a non-broadcast multi-access (NBMA) media. Since an OSPF broadcast media must support IP multicasting, a broadcast capable media, such as Ethernet, that does not support IP multicasting must be configured as a non-broadcast interface.

A non-broadcast interface supports any of the standard interface clauses listed above, plus the following two that are specific to non-broadcast interfaces:

pollinterval time

Before adjacency is established with a neighbor, OSPF packets are sent periodically at the specified pollinterval.

routers

By definition it is not possible to send broadcast packets to discover OSPF neighbors on a non-broadcast, so all neighbors must be configured. The list includes one or more neighbors and an indication of their eligibility to become a designated router.

virtuallink neighborid router_id transitarea area

Virtual links are used to establish or increase connectivity of the backbone area. The neighborid is the router_id of the other end of the virtual link. The transit area specified must also be configured on this system. All standard interface parameters defined by the interface clause above may be specified on a virtual link.

OSPF tracing options

lsabuild

Link State Advertisement creation

spf

Shortest Path First (SPF) calculations

dd

OSPF Database Description packets which are used in synchronizing OSPF databases.

request

OSPF Link State Request packets which are used in synchronizing OSPF databases.

lsu

OSPF Link State Update packets which are used in synchronizing OSPF databases.

ack

OSPF Link State Ack packets which are used in synchronizing OSPF databases.

The Exterior Gateway Protocol (EGP)

Before EGP sends routing information to a remote router, it must establish an adjacency with that router. This is accomplished by an exchange of Hello (not to be confused with the unsupported HELLO protocol, or OSPF HELLO messages) and I Heard You (I-H-U) messages with that router. Computers communicating via EGP are called EGP neighbors, and the exchange of HELLO and I-H-U messages is referred to as acquiring a neighbor. Once the neighbor is acquired, the system polls the neighbor for routing information. The neighbor responds by sending an update containing routing information. If the system receives a poll from its neighbor, it responds with its own update packet. When the system receives an update, it includes routes from the update into its routing database. If the neighbor fails to respond to three consecutive polls, gated assumes that the neighbor is down and removes the neighbor's routes from its database.

The EGP statement

preference preference

Sets the preference for routes learned from RIP. The default preference is 200. This preference may be overridden by a preference specified on the group or neighbor statements or by import policy.

defaultmetric metric ;

Defines the metric used when advertising routes via EGP. If not specified, the default value is 255 which some systems may consider unreachable. This choice of values requires you to explicitly specify a metric when exporting routes to EGP neighbors. This metric may be overridden by a metric specified on the neighbor or group statements or in export policy.

packetsize maxpacketsize

This defines the expected maximum size of a packet that EGP expects to receive from this neighbor. If a packet larger than this value is received, it will be incomplete and have to be discarded. The length of this packet will be noted and the expected size will be increased to be able to receive a packet of this size. Specifying the parameter here will prevent the first packet from being dropped. If not specified, the default size is 8192 bytes. All packet sizes are rounded up to a multiple of the system page size.

traceoptions trace_options

Specifies the tracing options for EGP. By default these are inherited from the global trace options. These values may be overridden on a group or neighbor basis. (See ``Trace statements'' and ``EGP tracing options''.)

group

EGP neighbors must be specified as members of a group. A group is usually used to group all neighbors in one autonomous system. Parameters specified on the group clause apply to all of the subsidiary neighbors unless explicitly overridden on a neighbor clause. Any number of group clauses may specify any number of neighbor clauses.

Any parameters from the neighbor subclause may be specified on the group clause to provide defaults for the whole group (which may be overridden for individual neighbors). In addition, the group clause is the only place to set the following attributes:

peeras

Identifies the autonomous system number expected from peers in the group. If not specified it will be learned dynamically.

localas

Identifies the autonomous system which gated is representing to the group. The default is that which has been set globally in the autonomoussystem statement. This option is usually only used when masquerading as another autonomous system and its use is discouraged.

maxup

Specifies the number of neighbors gated should acquire from this group. The default is to acquire all of the neighbors in the group. gated will attempt to acquire the first maxup neighbors in the order listed. If one of the first neighbors is not available, it will acquire one further down the list. If after start-up gated does manage to acquire the more desirable neighbor, it will drop the less desirable one.

neighbor neighbor_address

Each neighbor subclause defines one EGP neighbor within a group. The only part of the subclause that is required is the neighbor_address argument which is the symbolic host name or IP address of the neighbor. All other parameters are optional.

preference preference

Specifies the preference used for routes learned from these neighbors. This can differ from the default EGP preference set in the egp statement, so that gated can prefer routes from one neighbor, or group of neighbors, over another. This preference may be explicitly overridden by import policy.

preference2 preference

In the case of a preference tie, the second preference, preference2 may be used to break the tie. The default value is 0.

metricout metric

This defines a metric to be used for all routes sent to this neighbor. The value overrides the default metric set in the egp statement and any metrics specified by export policy, but only for this specific neighbor or group of neighbors.

nogendefault

Prevents gated from generating a default route when EGP receives a valid update from its neighbor. The default route is only generated when the gendefault option is enabled.

importdefault

Enables gated to accept the default route (0.0.0.0) if it is included in a received EGP update. If not specified, the default route contained in an EGP update is ignored. For efficiency, some networks have external routers announce a default route to avoid sending large EGP update packets.

exportdefault

Enables gated to include the default route (0.0.0.0) in EGP updates sent to this EGP neighbor. This allows the system to advertise the default route via EGP. Normally a default route is not included in EGP updates.

gateway gateway

If a network is not shared with a neighbor, gateway specifies a router on an attached network to be used as the next hop router for routes received from this neighbor. This option is only rarely used.

lcladdr local_address

Specifies the address to be used on the local end of the connection with the neighbor. The local address must be on an interface which is shared with the neighbor or with the neighbor's gateway when the gateway parameter is used. A session will only be opened when an interface with the appropriate local address (through which the neighbor or gateway address is directly reachable) is operating.

sourcenet network

Specifies the network queried in the EGP Poll packets. By default this is the network shared with neighbors address specified. If no network is shared with the neighbor, one of the networks to which the neighbor is attached should be specified. This parameter can also be used to specify a network shared with the neighbor other than the one on which the EGP packets are sent. This parameter is normally not needed.

p1 time

minhello time

Sets the minimum acceptable interval between the transmission of EGP HELLO packets. The default hello interval is 30 seconds. If the neighbor fails to respond to three hello packets, gated stops trying to acquire the neighbor. Setting a larger interval gives the neighbor a better chance to respond. minhello is an alias for the P1 value defined in the EGP specification.

p2 time

minpoll time

Sets the time interval between polls to the neighbor. The default is 120 seconds. If three polls are sent without a response, the neighbor is declared ``down'' and all routes learned from that neighbor are removed from the routing database. A longer polling interval supports a more stable routing database but is not as responsive to routing changes. minpoll is an alias for the P2 value defined in the EGP specification.

ttl ttl

By default, gated sets the IP TTL for local neighbors to one and the TTL for non-local neighbors to 255. This option is provided when attempting to communicate with improperly functioning routers that ignore packets sent with a TTL of one.

traceoptions trace_options

Specifies the tracing options for this EGP neighbor. By default these are inherited from group or EGP global trace options. (See ``Trace statements'' and ``EGP tracing options''.)

EGP tracing options

Packet tracing options (which may be modified with detail, send and recv):

packets

All EGP packets.

hello

EGP HELLO/I-HEARD-U packets which are used to determine neighbor reachability.

acquire

EGP ACQUIRE/CEASE packets which are used to initiate and terminate EGP sessions.

update

EGP POLL/UPDATE packets which are used to request and receive reachability updates.

The Border Gateway Protocol (BGP)

BGP supports two basic types of sessions between neighbors, internal (sometimes referred to as IBGP) and external. Internal sessions are run between routers in the same autonomous system, while external sessions run between routers in different autonomous systems. When sending routes to an external peer the local AS number is prepended to the AS path, hence routes received from an external peer are guaranteed to have the AS number of that peer at the start of the path. Routes received from an internal neighbor will not in general have the local AS number prepended to the AS path, and hence will in general have the same AS path that the route had when the originating internal neighbor received the route from an external peer. Routes with no AS numbers in the path may be legitimately received from internal neighbors; these indicate that the received route should be considered internal to your own AS.

The BGP implementation supports three versions of the BGP protocol, versions 2, 3 and 4. BGP versions 2 and 3 are quite similar in capability and function. They will only propagate classed network routes, and the AS path is a simple array of AS numbers. BGP 4 will propagate fully general address-and-mask routes, and the AS path has some structure to represent the results of aggregating dissimilar routes.

External BGP sessions may or may not include a single metric, which BGP calls the ``Multi-Exit Discriminator'', in the path attributes. For BGP versions 2 and 3 this metric is a 16-bit unsigned integer, for BGP version 4 it is a 32-bit unsigned integer. In either case smaller values of the metric are to be preferred. Currently this metric is only used to break ties between routes with equal preference from the same neighbor AS. Internal BGP sessions carry at least one metric in the path attributes, which BGP calls the ``LocalPref''. The size of the metric is identical to the MED. For BGP versions 2 and 3 this metric is considered better when its value is smaller, for version 4 it is better when it is larger. BGP version 4 sessions may optionally carry a second metric on internal sessions, this being an internal version of the Multi-Exit Discriminator. The use of these metrics is dependent on the type of internal protocol processing which is specified.

BGP collapses routes with similar path attributes into a single update for advertisement. Routes that are received in a single update will be readvertised in a single update. The churn caused by the loss of a neighbor will be minimized and the initial advertisement sent during peer establishment will be maximally compressed. BGP does not read information from the kernel message-by-message, but fills the input buffer. It processes all complete messages in the buffer before reading again. BGP also does multiple reads to clear all incoming data queued on the socket. This feature may cause other protocols to be blocked for prolonged intervals by a busy peer connection.

All unreachable messages are collected into a single message and sent prior to reachable routes during a flash update. For these unreachable announcements, the next hop is set to the local address on the connection, no metric is sent and the path origin is set to incomplete. On external connections the AS path in unreachable announcements is set to the local AS, on internal connections the AS path is set to zero length.

The BGP implementation expects external peers to be directly attached to a shared subnet, and expects those peers to advertise next hops which are host addresses on that subnet (though this constraint can be relaxed by configuration for testing). For groups of internal peers, however, there are several alternatives which may be selected from by specifying the group type. Type internal groups expect all peers to be directly attached to a shared subnet so that, like external peers, the next hops received in BGP advertisements may be used directly for forwarding. Type routing groups instead will determine the immediate next hops for routes by using the next hop received with a route from a peer as a forwarding address, and using this to look up an immediate next hop in an IGP's routes. Such groups support distant peers, but need to be informed of the IGP whose routes they are using to determine immediate next hops. Finally, type igp groups expect routes from the group peers to not be used for forwarding at all. Instead they expect that copies of the BGP routes received will also be received via an IGP, and that the BGP routes will only be used to determine the path attributes associated with the IGP routes. Such groups also support distant peers, and also need to be informed of the IGP they are running with.

For internal BGP group types (and for test groups), where possible a single outgoing message is built for all group peers based on the common policy. A copy of the message is sent to every peer in the group, with possible adjustments to the next hop field as appropriate to each peer. This minimizes the computational load of running large numbers of peers in these types of groups. BGP allows unconfigured peers to connect if an appropriate group has been configured with an allow clause.

The BGP statement

The bgp statement enables or disables BGP. By default BGP is disabled. The default metric for announcing routes via BGP is not to send a metric.

preference preference

Sets the preference for routes learned from RIP. The default preference is 170. This preference may be overridden by a preference specified on the group or peer statements or by import policy.

defaultmetric metric

Defines the metric used when advertising routes via BGP. If not specified, no metric is propagated. This metric may be overridden by a metric specified on the neighbor or group statements or in export policy.

traceoptions trace_options

Specifies the tracing options for BGP. By default these are inherited from the global trace options. These values may be overridden on a group or neighbor basis. (See ``Trace statements'' and ``BGP tracing options''.)

Groups

group type external peeras autonomous_system

In the classic external BGP group, full policy checking is applied to all incoming and outgoing advertisements. The external neighbors must be directly reachable through one of the machine's local interfaces. By default no metric is included in external advertisements, and the next hop is computed with respect to the shared interface.

group type internal peeras autonomous_system

An internal group operating where there is no IP-level IGP, for example an SMDS network or MILNET. All neighbors in this group are required to be directly reachable via a single interface. All next hop information is computed with respect to this interface. Import and export policy may be applied to group advertisements. Routes received from external BGP or EGP neighbors are by default readvertised with the received metric.

group type igp peeras autonomous_system proto proto

An internal group that runs in association with an interior protocol. The IGP group examines routes which the IGP is exporting and sends an advertisement only if the path attributes could not be entirely represented in the IGP tag mechanism. Only the AS path, path origin, and transitive optional attributes are sent with routes. No metric is sent, and the next hop is set to the local address used by the connection. Received internal BGP routes are not used or readvertised. Instead, the AS path information is attached to the corresponding IGP route and the latter is used for readvertisement. Since internal IGP peers are sent only a subset of the routes which the IGP is exporting, the export policy used is the IGP's. There is no need to implement the ``do not routes from peers in the same group'' constraint since the advertised routes are routes that IGP already exports.

group type routing peeras autonomous_system proto proto interface interface_list

An internal group which uses the routes of an interior protocol to resolve forwarding addresses. A ``type routing'' group propagates external routes between routers which are not directly connected, and computes immediate next hops for these routes by using the BGP next hop which arrived with the route as a forwarding address to be resolved via an internal protocol's routing information. In essence, internal BGP is used to carry AS external routes, while the IGP is expected to only carry AS internal routes, and the latter is used to find immediate next hops for the former.

The proto names the interior protocol to be used to resolve BGP route next hops, and may be the name of any IGP in the configuration. By default the next hop in BGP routes advertised to type routing peers will be set to the local address on the BGP connection to those peers, as it is assumed a route to this address will be propagated via the IGP. The interface_list can optionally provide a list interfaces whose routes are carried via the IGP for which third party next hops may be used instead.

group type test peeras autonomous_system

An extension to external BGP which implements a fixed policy using test peers. Fixed policy and special case code make test peers relatively inexpensive to maintain. Test peers do not need to be on a directly attached network. If gated and the peer are on the same (directly attached) subnet, the advertised next hop is computed with respect to that network, otherwise the next hop is the local machine's current next hop. All routing information advertised by and received from a test peer is discarded, and all BGP advertizable routes are sent back to the test peer. Metrics from EGP- and BGP-derived routes are forwarded in the advertisement, otherwise no metric is included.

Group parameters

Specifying peers

allow

The allow clauses allows for peer connections from any addresses in the specified range of network and mask pairs. All parameters for these peers must be configured on the group clause. The internal peer structures are created when an incoming open request is received and destroyed when the connection is broken. For more information about specifying network/mask pairs, see ``Route filtering''.

peer host

A peer clause configures an individual peer. Each peer inherits all parameters specified on a group as defaults. Those default may be overridden by parameters explicitly specified on the peer subclause.

Within each group clause, individual peers can be specified or a group of potential peers can be specified using allow. allow is used to specify a set of address masks. If gated receives a BGP connection request from any address in the set specified, it will accept it and set up a peer relationship.

Peer parameters

The BGP peer subclause allows the following parameters, which can also be specified on the group clause. All are optional.

metricout metric

If specified, this metric is used as the primary metric on all routes sent to the specified peer(s). This metric overrides the default metric, a metric specified on the group and any metric specified by export policy.

localas autonomous_system

Identifies the autonomous system which gated is representing to this group of peers. The default is that which has been set globally in the autonomoussystem statement.

nogendefault

Prevents gated from generating a default route when EGP receives a valid update from its neighbor. The default route is only generated when the gendefault option is enabled.

gateway gateway

If a network is not shared with a peer, gateway specifies a router on an attached network to be used as the next hop router for routes received from this neighbor. This parameter is not needed in most cases.

preference preference

Specifies the preference used for routes learned from these peers. This can differ from the default BGP preference set in the bgp statement, so that gated can prefer routes from one peer, or group of peer, over others. This preference may be explicitly overridden by import policy.

preference2 preference

In the case of a preference tie, the second preference, preference2 may be used to break the tie. The default value is 0.

lcladdr local_address

Specifies the address to be used on the local end of the TCP connection with the peer. For external peers the local address must be on an interface which is shared with the peer or with the peer's gateway when the gateway parameter is used. A session with an external peer will only be opened when an interface with the appropriate local address (through which the peer or gateway address is directly reachable) is operating. For other types of peers, a peer session will be maintained when any interface with the specified local address is operating. In either case incoming connections will only be recognized as matching a configured peer if they are addressed to the configured local address.

holdtime time

Specifies the BGP holdtime value to use when negotiating the connection with this peer, in seconds. According to BGP, if gated does not receive a keepalive, update, or notification message within the period specified in the Hold Time field of the BGP Open message, then the BGP connection will be closed. The value must be either 0 (no keepalives will be sent) or at least 3.

version version

Specifies the version of the BGP protocol to use with this peer. If not specified, the highest supported version is used first and version negotiation is attempted. If it is specified, only the specified version will be offered during negotiation. Currently supported version are 2, 3 and 4.

passive

Specifies that active OPENs to this peer should not be attempted. gated should wait for the peer to issue an open. By default all explicitly configured peers are active, they periodically send OPEN messages until the peer responds.

sendbuffer buffer_size

recvbuffer buffer_size

Control the amount of send and receive buffering asked of the kernel. The maximum supported is 65535 bytes although many kernels have a lower limit. By default, gated configures the maximum supported. These parameters are not needed on normally functioning systems.

indelay time

outdelay time

Used to dampen route fluctuations. indelay is the amount of time a route learned from a BGP peer must be stable before it is accepted into the gated routing database. outdelay is the amount of time a route must be present in the gated routing database before it is exported to BGP. The default value for each is 0, meaning that these features are disabled.

keep all

Used to retain routes learned from a peer even if the routes' AS paths contain one of our exported AS numbers.

analretentive

Causes gated to issue warning messages when receiving questionable BGP updates such as duplicate routes and/or deletions of non-existing routes. Normally these events are silently ignored.

noauthcheck

Normally gated verifies that incoming packets have an authentication field of all ones. This option may be used to allow communication with an implementation that uses some other form of authentication.

noaggregatorid

Causes gated to specify the routerid in the aggregator attribute as zero (instead of its routerid) in order to prevent different routers in an AS from creating aggregate routes with different AS paths.

keepalivesalways

Causes gated always to send keepalives, even when an update could have correctly substituted for one. This allows interoperability with routers that do not completely obey the protocol specifications on this point.

v3asloopokay

By default gated will not advertise routes whose AS path is looped (that is, with an AS appearing more than once in the path) to version 3 external peers. Setting this flag removes this constraint. Ignored when set on internal groups or peers.

nov4asloop

Prevents routes with looped AS paths from being advertised to version 4 external peers. This can be useful to avoid advertising such routes to peer which would incorrectly forward the routes on to version 3 neighbors.

logupdown

Causes a message to be logged via the syslog mechanism whenever a BGP peer enters or leaves the ESTABLISHED state.

ttl ttl

By default, gated sets the IP TTL for local peers to one and the TTL for non-local peers to 255. This option mainly is provided when attempting to communicate with improperly functioning routers that ignore packets sent with a TTL of one.

traceoptions trace_options

Specifies the tracing options for this BGP neighbor. By default these are inherited from group or BGP global trace options. (See ``Trace statements'' and ``BGP tracing options''.)

BGP tracing options

Packet tracing options (which may be modified with detail, send and recv):

packets

All BGP packets

open

BGP OPEN packets which are used to establish a peer relationship.

update

BGP UPDATE packets which are used to pass network reachability information.

keepalive

BGP KEEPALIVE packets which are used to verify peer reachability.

The ICMP statement

The ICMP statement syntax is:

icmp {
traceoptions trace_options ;
}

traceoptions trace_options ;

Specifies the tracing options for ICMP. (See ``Trace statements'' and ``ICMP tracing options''.)

ICMP tracing options

packets

All ICMP packets received.

redirect

Only ICMP REDIRECT packets received.

routerdiscovery

Only ICMP ROUTER DISCOVERY packets received.

info

Only ICMP informational packets, which include mask request/response, info request/response, echo request/response and time stamp request/response.

error

Only ICMP error packets, which include time exceeded, parameter problem, unreachable and source quench.

Redirect processing

If gated determines that a redirect is not acceptable, it tries to figure out if the kernel forwarding table has been modified. On systems where ICMP messages are monitored this is accomplished by trying to second guess what the kernel would have done with the redirect.

If gated has determined that the state of the kernel forwarding table has been changed, the necessary requests to the kernel are made to restore the correct state.

Note that on currently available systems it is not possible to disable the processing of ICMP redirects, even when the system is functioning as a router. To ignore the effects of redirects, gated must process each one and actively restore any changes it made to the kernel's state. Because of the mechanism's involved there will be windows where the effects of redirects are present in the kernel.

By default, gated removes redirects when actively participating in an interior routing protocol (RIP or OSPF). It is not possible to enable redirects once they have been automatically disabled. Listening to RIP in nobroadcast mode does not cause redirects to be ignored, nor does the use of EGP and BGP. Redirects must be manually configured off in these cases.

Note that in accordance with the latest IETF Router Requirements document, gated insures that all ICMP net redirects are processed as host redirects. When an ICMP net redirect is accepted, gated issues the requests to the kernel to make sure that the kernel forwarding table is updated to reflect a host redirect instead of a net redirect.

The redirect statement does not prevent the system from sending redirects, only from listening to them.

The redirect statement

preference

Sets the preference for a route learned from a redirect. The default is 30.

interface interface_list

The interface statement allows the enabling and disabling of redirects on an interface-by-interface basis. See ``Interface list specification'' for the description of the format of interface_list. The possible parameters are:

noredirects

Specifies that redirects received via the specified interface will be ignored. The default is to accept redirects on all interfaces.

redirects

This is the default. This argument may be necessary when noredirects is used on a wildcard interface descriptor.

trustedgateways gateway_list

Defines the list of gateways from which redirects will be accepted. The gateway_list is simply a list of host names or addresses. By default, all routers on the shared network(s) are trusted to supply redirects. But if the trustedgateways clause is specified only redirects from the gateways in the list are accepted.

traceoptions trace_options

There are no redirect-specific tracing options. All non-error messages are traced under the normal class.

Tracing options

The Router Discovery Protocol

The protocol is split into two portions, the server portion which runs on routers, and the client portion that runs on hosts. gated treats these much like two separate protocols, only one of which may be enabled at a time.

The Router Discovery server

Initially these Router Advertisements occur every few seconds, then fall back to every few minutes. In addition, a host may send a ROUTER SOLICITATION to which the router will respond with a unicast Router Advertisement (unless a multicast or broadcast advertisement is due momentarily).

Each Router Advertisement contains an Advertisement Lifetime field indicating for how long the advertised addresses are valid. This lifetime is configured such that another Router Advertisement will be sent before the lifetime has expired. A lifetime of zero is used to indicate that one or more addresses are no longer valid.

On systems supporting IP multicasting, the Router Advertisements are by default send to the all-hosts multicast address 224.0.0.1. However, the use of broadcast may be specified. When Router Advertisements are being sent to the all-hosts multicast address, or an interface is configured for the limited-broadcast address 255.255.255.255, all IP addresses configured on the physical interface are included in the Router Advertisement. When the Router advertisements are being sent to a net or subnet broadcast, only the address associated with that net or subnet is included.

The Router Discovery server statement

traceoptions trace_options

Specifies the Router Discovery tracing options. (See ``Trace statements'' and ``Router Discovery tracing options''.)

interface interface_list

Specifies the parameters that apply to physical interfaces. Note a slight difference in convention from the rest of gated, interface specifies just physical interfaces (such as le0, ef0 and en1), while address specifies protocol (in this case IP) addresses.

Interface parameters are:

maxadvinterval time

The maximum time allowed between sending broadcast or multicast Router Advertisements from the interface. Must be no less than 4 and no more than 30:00 (30 minutes or 1800 seconds). The default is 10:00 (10 minutes or 600 seconds).

minadvinterval time

The minimum time allowed between sending unsolicited broadcast or multicast Router Advertisements from the interface. Must be no less than 3 seconds and no greater than maxadvinterval. The default is 0.75 * maxadvinterval.

lifetime time

The lifetime of addresses in a Router Advertisement. Must be no less than maxadvinterval and no greater than 2:30:00 (two hours, thirty minutes or 9000 seconds). The default is 3 * maxadvinterval.

address interface_list

Specifies the parameters that apply to the specified set of addresses on this physical interfaces. Note a slight difference in convention from the rest of gated, interface specifies just physical interfaces (such as le0, ef0 and en1), while address specifies protocol (in this case IP) addresses.

advertise

Specifies that the specified address(es) should be included in Router Advertisements. This is the default.

ignore

Specifies that the specified address(es) should not be included in Router Advertisements.

broadcast

Specifies that the given address(es) should be included in a broadcast Router Advertisement because this system does not support IP multicasting, or some hosts on attached network do not support IP multicasting. It is possible to mix addresses on a physical interface such that some are included in a broadcast Router Advertisement and some are included in a multicast Router Advertisement. This is the default if the router does not support IP multicasting.

multicast

Specifies that the given address(es) should only be included in a multicast Router Advertisement. If the system does not support IP multicasting the address(es) will not be included. If the system supports IP multicasting, the default is to include the address(es) in a multicast Router Advertisement if the given interface supports IP multicasting, if not the address(es) will be included in a broadcast Router Advertisement.

preference preference

The preferability of the address(es) as a default router address, relative to other router addresses on the same subnet. A 32-bit, signed, twos-complement integer, with higher values meaning more preferable. Note that hex 80000000 may only be specified as ineligible. The default is 0.

ineligible

Specifies that the given address(es) will be assigned a preference of (hex 80000000) which means that it is not eligible to be the default route for any hosts.

This is useful when the address(es) should not be used as a default route, but are given as the next hop in an ICMP redirect. This allows the hosts to verify that the given addresses are up and available.

The Router Discovery client

When a Router Advertisement with non-zero lifetime is received, the host installs a default route to each of the advertised addresses. If the preference ineligible, or the address is not on an attached interface, the route is marked unusable but retained. If the preference is usable, the metric is set as a function of the preference such that the route with the best preference is used. If more than one address with the same preference is received, the one with the lowest IP address will be used. These default routes are not exportable to other protocols.

When a Router Advertisement with a zero lifetime is received, the host deletes all routes with next-hop addresses learned from that router. In addition, any routers learned from ICMP redirects pointing to these addresses will be deleted. The same will happen when a Router Advertisement is not received to refresh these routes before the lifetime expires.

The Router Discovery client statement

traceoptions trace_options

Specifies the tracing options for router discovery. (See ``Trace statements'' and ``Router Discovery tracing options''.)

preference preference ;

Specifies the preference of all Router Discovery default routes. The default is 55.

interface interface_list

Specifies the parameters that apply to physical interfaces. Note a slight difference in convention from the rest of gated, interface specifies just physical interfaces (such as le0, ef0 and en1). The Router Discovery Client has no parameters that apply only to interface addresses.

enable

Specifies that Router Discovery should be performed on the specified interface(s). This is the default.

disable

Specifies that Router Discovery should not be performed on the specified interface(s).

broadcast

Specifies that Router Solicitations should be broadcast on the specified interface(s). This is the default if IP multicast support is not available on this host or interface.

multicast

Specifies that Router Solicitations should be multicast on the specified interface(s). If IP multicast is not available on this host and interface, no solicitation will be performed. The default is to multicast Router Solicitations if the host and interface support it, otherwise Router Solicitations are broadcast.

quiet

Specifies that no Router Solicitations will be sent on this interface, even though Router Discovery will be performed.

solicit

Specifies that initial Router Solicitations will be sent on this interface. This is the default.

Router Discovery tracing options

state

State transitions

The SNMP statement

gated communicates with the SNMP daemon via the SMUX protocol that is described in RFC 1227.

The SNMP statement syntax is:

snmp yes | no | on | off
[ {
port port ;
debug ;
traceoptions traceoptions ;
} ] ;

Reporting is enabled by specifying yes or on and disabled with no or off.

The default is on.

port port

By default, the SMUX daemon listens for requests on port 199. gated can be configured to try to contact the SMUX daemon on a different port by explicitly specifying the port.

debug

Specifying this option enables debugging of the SMUX code. The default is debugging disabled.

traceoptions trace_options

Specifies the tracing options for SMUX. (See ``Trace statements'' and ``SMUX tracing options''.)

SMUX tracing options

receive

SNMP requests received from the SMUX daemon and the associated responses.

register

Protocol requests to register variables.

resolve

Protocol requests to resolve variable names.

trap

SNMP trap requests from protocols.

The kernel statement

The add, delete and change operations gated must use to update the typical kernel forwarding table take a non-trivial amount of time. This does not present a problem for older routing protocols (RIP, EGP), which are not particularly time critical and do not easily handle very large numbers of routes anyway. The newer routing protocols (OSPF, BGP) have stricter timing requirements and are often used to process many more routes. The speed of the kernel interface becomes critical when these protocols are used.

To prevent gated from locking up for significant periods of time installing large numbers of routes (up to a minute or more has been observed on real networks), the processing of these routes is now done in batches. The size of these batches may be controlled by the tuning parameters described below, but normally the default parameters will provide the proper functionality.

During normal shutdown processing, gated normally deletes all the routes it has installed in the kernel forwarding table, except for those marked with retain. Optionally, gated can leave all routes in the kernel forwarding table by not deleting any routes. In this case changes will be made to insure that routes with a retain indication are installed in the table. This is useful on systems with large numbers of routes as it prevents the need to re-install the routes when gated restarts. This can greatly reduce the time it takes to recover from a restart.

Forwarding tables and routing tables

Kernel configuration

options option_list

Configure kernel options. The valid options are:

nochange

On systems supporting the routing socket this insures that changes operations will not be performed, only deletes and adds.

noflushatexit

During normal shutdown processing gated deletes all routes from the kernel forwarding table that do not have a retain indication. The noflushatexit option prevents route deletions at shutdown. Instead, routes are changed and added to make sure that all the routes marked with retain get installed.

This is handy on systems with thousands of routes. Upon startup gated will notice which routes are in the kernel forwarding table and not have to add them back.

remnantholddimte time

Normally remnant routes read from the kernel forwarding table at startup are timed out in three minutes or as soon as they are overridden. This option allows the interval to be configured to a value between zero and 15 minutes. Setting it to zero causes these routes to be deleted immediately.

routes number

On some systems kernel memory is at a premium. With this parameter a limit can be placed on the maximum number of routes gated will install in the kernel. Normally gated adds/changes/deletes routes in interface/internal/external order, that is, it queues interface routes first, followed by internal routes, followed by external routes, and processes the queue from the beginning. If this parameter is specified and the limit is hit, gated does two scans of the list instead. On the first scan it does deletes, and also deletes all changed routes, turning the queued changes into adds. It then rescans the list doing adds in interface/internal/external order until it hits the limit again. This will tend to favor internal routes over external routes. The default is not to limit the number of routes in the kernel forwarding table.

flash

When routes change, the process of notifying the protocols is called a flash update. The kernel forwarding table interface is the first to be notified. Normally a maximum of 20 interface routes may be processed during one flash update. The flash command allows tuning of these parameters:

limit number

Specifies the maximum number of routes which may be processed during one flash update. The default is 20. A value of -1 will cause all pending route changes of the specified type to be processed during the flash update.

type interface | interior | all

Specifies the type of routes that will be processed during a flash update. interior specifies that interior routes (see ``Interior routing protocols'') will also be installed. all specifies the inclusion of exterior routes as well (see ``Exterior routing protocols''). The default is interface which specifies that only interface routes will be installed during a flash update.

Specifying flash limit -1 all causes all routes to be installed during the flash update; this mimics the behavior of previous versions of gated.

background

Since only interface routes are normally installed during a flash update, the remaining routes are processed in batches in the background, that is, when no routing protocol traffic is being received. Normally, 120 routes are installed at a time to allow other tasks to be performed and this background processing is done at lower priority than flash updates the following parameters allow tuning of these parameters:

limit number

Specifies the number of route which may be processed at during one batch. The default is 120.

priority flash | higher | lower

Specifies the priority of the processing of batches of kernel updates in relationship to the flash update processing. The default is lower which means that flash updates are processed first. To process kernel updates at the same priority as flash updates, specify flash; to process them at a lower priority, use lower.

Tracing options

symbols

Symbols read from the kernel, by nlist or similar interface.

iflist

Interface list scan. This option is useful when entered from the command line as the first interface list scan is performed before the configuration file is parsed.

The following tracing options may only be specified in the configuration file. They are not valid from the command line.

remnants

Routes read from the kernel when gated starts.

request

Requests by gated to Add/Delete/Change routes in the kernel forwarding table.

Static statements

host host gateway gateway_list

( network [ ( mask mask ) | ( masklen number ) ] )

default gateway gateway_list

This is the most general form of the static statement. It defines a static route through one or more gateways. Static routes are installed when one or more of the gateways listed are available on directly attached interfaces. If more than one eligible gateways are available, they are limited by the number of multipath destinations supported. Currently, this number is one.

Parameters for static routes are:

interface interface_list

When this parameter is specified, gateways are only considered valid when they are on one of these interfaces. See ``Interface list specification'' for a description of the format of interface_list.

preference preference

This option selects the preference of this static route. The preference controls how this route competes with routes from other protocols. The default preference is 60.

retain

Normally gated removes all routes except interface routes from the kernel forwarding table during a graceful shutdown. The retain option may be used to prevent specific static routes from being removed. This is useful to insure that some routing is available when gated is not running.

reject

Instead of forwarding a packet like a normal route, reject routes cause packets to be dropped and unreachable messages to be sent to the packet originators. Specifying this option causes this route to be installed as a reject route. Not all kernel forwarding engines support reject routes.

blackhole

A blackhole route is the same as a reject route except that unreachable messages are not supported.

noinstall

Normally the route with the lowest preference is installed in the kernel forwarding table and is the route exported to other protocols. When noinstall is specified on a route, it will not be installed in the kernel forwarding table when it is active, but it will still be eligible to be exported to other protocols.

( network [ ( mask mask ) | ( masklen number ) ] ) interface interface

This form defines a static interface route which is used for primitive support of multiple network addresses on one interface. The preference, retain, reject, blackhole and noinstall options are the same as described above.

Route filtering

The action taken when no match is found is dependent on the context, for instance import and export route filters assume an all reject ; at the end a list.

A route will match the most specific filter that applies. Specifying more than one filter with the same destination, mask and modifiers will generate an error.

Filtering syntax

These are all the possible formats for a route filter. Not all of these formats are available in all places, for instance the host and default formats are not valid for martians.

In most cases it is possible to specify additional parameters relevant to the context of the filter. For example, on a martian statement it is possible to specify the allow keyword, on an import statement you can specify a preference, and on a export you can specify a metric.

network [ exact | refines ]

network mask mask [ exact | refines ]

network masklen number [ exact | refines ]

Matching usually requires both an address and a mask, although the mask is implied in the shorthand forms listed below. These three forms vary in how the mask is specified. In the first form, the mask is implied to be the natural mask of the network. In the second, the mask is explicitly specified. In the third, the mask is specified by the number of contiguous one bits.

If no additional parameters are specified, any destination that falls in the range given by the network and mask is matched, the mask of the destination is ignored. If a natural network is specified, the network, any subnets, and any hosts will be match. The two optional modifiers cause the mask of the destination to be considered also:

exact

This parameter specifies that the mask of the destination must match the supplied mask exactly. This is used to match a network, but no subnets or hosts of that network.

refines

Specifies that the mask of the destination must be longer than the filter mask. This is used to match subnets and/or hosts of a network, but not the network.

all

This entry matches anything. It is equivalent to 0.0.0.0 mask 0.0.0.0.

default

Matches the default route. To match, the address must be the default address and the mask must be all zeros. This is equivalent to: 0.0.0.0 mask 0.0.0.0 exact.

host host

Matches the specific host. To match, the address must exactly match the specified host and the network mask must be a host mask (that is, all ones). This is equivalent to host mask 255.255.255.255 exact.

Matching AS paths

Each autonomous system that a route passed through prepends its AS number to the beginning of the AS path.

The origin information details the completeness of AS path information. An origin of igp indicates the route was learned from an interior routing protocol and is most likely complete. An origin of egp indicates the route was learned from an exterior routing protocol that does not support AS paths (EGP, for example) and the path is most likely not complete. When the path information is definitely not complete, an origin of incomplete is used.

AS path regular expressions are defined in RFC 1164 section 4.2.

AS path matching syntax

This specifies that an AS matching the aspath_regexp with the specified origin is matched.

AS path regular expressions

AS path terms

autonomous_system

Is any valid autonomous system number, from one through 65534 inclusive.

.

Matches any autonomous system number.

( aspath_regexp )

Parentheses group subexpressions -- an operator, such as * or ? works on a single element or on a regular expression enclosed in parentheses.

AS path operators

aspath_term {m,n}

a regular expression followed by {m,n} (where m and n are both non-negative integers and m <= n) means at least m and at most n repetitions.

aspath_term {m}

a regular expression followed by {m} (where m is a positive integer) means exactly m repetitions.

aspath_term {m,}

a regular expression followed by {m,} (where m is a positive integer) means m or more repetitions.

aspath_term *

an AS path term followed by * means zero or more repetitions. This is shorthand for {0,}.

aspath_term +

a regular expression followed by + means one or more repetitions. This is shorthand for {1,}.

aspath_term ?

a regular expression followed by ? means zero or one repetition. This is shorthand for {0,1}.

aspath_term | aspath_term

matches the AS term on the left, or the AS term on the right.

The import statement

Specifying preferences

restrict

Specifies that the routes are not desired in the routing table. In some cases this means that the routes are not installed in the routing table. In others it means that they are installed with a negative preference; this prevents them from becoming active so they will not be installed in the forwarding table, or exported to other protocols.

preference preference

Specifies the preference value used when comparing this route to other routes from other protocols. The route with the lowest preference available at any given route becomes the active route, is installed in the forwarding table, and is eligible to be exported to other protocols. The default preferences are configured by the individual protocols.

Route filters

See ``Route filtering'' for a detailed explanation of how route filtering works. When no route filtering is specified (that is, when restrict is specified on the first line of a statement), all routes from the specified source will match that statement. If any filters are specified, only routes that match the specified filters will be imported. Put differently, if any filters are specified, an all restrict ; is assumed at the end of the list.

Importing routes from BGP and EGP

EGP importation may be controlled by autonomous system. BGP also supports controlling propagation by the use of an AS path regular expressions, which are documented in ``Matching AS paths''. Note that EGP and BGP versions 2 and 3 only support the propagation of natural networks, so the host and default route filters are meaningless. BGP version 4 supports the propagation of any destination along with a contiguous network mask.

EGP and BGP both store any routes that were rejected implicitly by not being mentioned in a route filter, or explicitly with the restrict keyword in the routing table with a negative preference. A negative preference prevents a route from becoming active, which prevents it from being installed in the forwarding table, or exported to other protocols. This alleviates the need to break and re-establish a session upon reconfiguration if importation policy is changed.

Importing routes from an RIP and Redirects

The importation of RIP and Redirect routes may be controlled by any of protocol, source interface and source gateway. If more than one is specified, they are processed from most general (protocol) to most specific (gateway).

RIP does not support the use of preference to choose between routes of the same protocol. That is left to the protocol metrics. These protocols do not save routes that were rejected since they have short update intervals.

Importing routes from OSPF

Due to the nature of OSPF, only the importation of ASE routes may be controlled. OSPF intra- and inter-area routes are always imported into the gated routing table with a preference of 10. If a tag is specified, the import clause will only apply to routes with the specified tag.

It is only possible to restrict the importation of OSPF ASE routes when functioning as an AS border router. This is accomplished by specifying an export ospfase clause. Specification of an empty export clause may be used to restrict importation of ASEs when no ASEs are being exported.

Like the other interior protocols, preference can not be used to choose between OSPF ASE routes, that is done by the OSPF costs. Routes that are rejected by policy are stored in the table with a negative preference.

The export statement

The outer portion of a given export statement specifies the destination of the routing information you are controlling. The middle portion restricts the sources of importation that you wish to consider. And the innermost portion is a route filter used to select individual routes.

Specifying metrics

restrict

Specifies that nothing should be exported. If specified on the destination portion of the export statement it specifies that nothing at all should be exported to this destination. If specified on the source portion it specifies that nothing from this source should be exported to this destination. If specified as part of a route filter, it specifies that the routes matching that filter should not be exported.

metric metric

Specifies the metric to be used when exporting to the specified destination.

Route filters

See ``Route filtering'' for a detailed explanation of how route filtering works. When no route filtering is specified (that is, when restrict is specified on the first line of a statement), all routes from the specified source will match that statement. If any filters are specified, only routes that match the specified filters will be exported. Put differently, if any filters are specified, an all restrict ; is assumed at the end of the list.

Specifying the destination

The specification of the source of the routing information being exported (the export list) is described below.

Exporting to EGP and BGP

Exportation to EGP and BGP is controlled by autonomous system, the same policy is applied to all routers in the AS. EGP metrics range from 0 to 255 inclusive with 0 being the most attractive.

BGP metrics are 16-bit unsigned quantities, that is, they range from 0 to 65535 inclusive with 0 being the most attractive. While BGP version 4 actually supports 32-bit unsigned quantities, gated does not yet support this.

If no export policy is specified, only routes to attached interfaces will be exported. If any policy is specified the defaults are overridden; it is necessary to explicitly specify everything that should be exported.

Note that EGP and BGP versions 2 and 3 only support the propagation of natural networks, so the host and default route filters are meaningless. BGP version 4 supports the propagation of any destination along with a contiguous network mask.

Exporting to RIP

Exportation to RIP is controlled by any of protocol, interface or gateway. If more than one is specified, they are processed from most general (protocol) to most specific (gateway).

It is not possible to set metrics for exporting RIP routes into RIP. Attempts to do this are silently ignored.

If no export policy is specified, RIP and interface routes are exported into RIP. If any policy is specified the defaults are overridden; it is necessary to explicitly specify everything that should be exports.

RIP version 1 assumes that all subnets of the shared network have the same subnet mask so they are only able to propagate subnets of that network. RIP version 2 removes that restriction and is capable of propagating all routes when not sending version 1 compatible updates.

To announce routes which specify a next hop of the loopback interface (that is, static and internally generated default routes) via RIP, it is necessary to specify the metric at some level in the export clause. Just setting a default metric for RIP is not sufficient. This is a safeguard to verify that the announcement is intended.

Exporting to OSPF

It is not possible to create OSPF intra- or inter-area routes by exporting routes from the gated routing table into OSPF. It is only possible to export from the gated routing table into OSPF ASE routes. It is also not possible to control the propagation of OSPF routes within the OSPF protocol.

There are two types of OSPF ASE routes, type 1 and type 2, see the OSPF protocol configuration for a detailed explanation of the two types. The default type is specified by the defaults subclause of the ospf clause. This may be overridden by a specification on the export statement.

OSPF ASE routes also have the provision to carry a tag. This is an arbitrary 32-bit number that can be used on OSPF routers to filter routing information. See ``The OSPF statement'' for detailed information on OSPF tags. The default tag specified by the ospf defaults clause may be overridden by a tag specified on the export statement.

Specifying the source

Exporting BGP and EGP routes

BGP and EGP routes may be specified by source autonomous system. All routes may be exported by as path, see below for more information.

Exporting RIP routes

RIP routes may be exported by protocol, source interface and/or source gateway.

Exporting OSPF routes

Both OSPF, and OSPF ASE routes may be exported into other protocols. See ``Exporting by route tag'' for information on exporting by tag.

Exporting routes from non-routing protocols

The non-routing ``protocols'' are:

direct

Routes to directly attached interfaces.

static

Static routes specified in a static clause.

kernel

On systems with the routing socket, routes learned from the routing socket are installed in the gated routing table with a protocol of kernel. These routes may be exported by referencing this protocol. This is useful when it is desirable to have a script install routes with the route command and propagate them to other routing protocols.

The non-routing ``protocols'' are:

default

Refers to routes created by the gendefault option. It is recommended that route generation be used instead.

aggregate

Refers to routes synthesized from other routes when the aggregate and generate statements are used. See ``Route aggregation'' for more information.

Exporting by AS path

When BGP is configured, all routes are assigned an AS path when they are added to the routing table. For all interior routes this AS path specifies IGP as the origin and no ASes in the AS path (the current AS is added when the route is exported). For EGP routes this AS path specifies EGP as the origin and the source AS as the AS path. For BGP routes, the AS path is stored as learned from BGP.

AS path regular expressions are documented in ``Matching AS paths''.

Exporting by route tag

Both OSPF and RIP version 2 currently support tags, all other protocols always have a tag of zero. The source of exported routes may be selected based on this tag. This is useful when routes are classified by tag when the are exported into a given routing protocol.

Route aggregation

Route aggregation is also used by regional and national networks to reduce the amount of routing information passed around. With careful allocation of network addresses to clients, regional networks can just announce one route to regional networks instead of hundreds.

Aggregate routes are not actually used for packet forwarding by the originator of the aggregate route, only by the receiver (if it wishes). A router receiving a packet which does not match one of the component routes which led to the generation of an aggregate route is supposed to respond with an ICMP network unreachable message. This is to prevent packets for unknown component routes from following a default route into another network where they would be forwarded back to the border router, and around and around again and again, until their TTL expires. Sending an unreachable message for a missing piece of an aggregate is only possible on systems with support for reject routes.

A slight variation of aggregation is the generation of a route based on the existence of certain conditions. This is sometimes known as the route of last resort. This route inherits the next hops and aspath from the contributor specified with the lowest (most favorable) preference. The most common usage for this is to generate a default based on the presence of a route from a peer on a neighboring backbone.

Aggregation and generation syntax

Routes that match the route filters are called contributing routes. They are ordered according to the aggregation preference that applies to them. If there are more than one contributing routes with the same aggregating preference, the route's own preferences are used to order the routes. The preference of the aggregate route will be that of contributing route with the lowest aggregate preference.

preference preference

Specifies the preference to assign to the resulting aggregate route. The default preference is 130.

brief

Used to specify that the AS path should be truncated to the longest common AS path. The default is to build an AS path consisting of SETs and SEQUENCE of all contributing AS paths.

proto proto

In addition to the special protocols listed, the contributing protocol may be chosen from among any of the ones supported (and currently configured into) gated.

as autonomous_system

Restrict selection of routes to those learned from the specified autonomous system.

tag tag

Restrict selection of routes to those with the specified tag.

aspath aspath_regexp

Restrict selection of routes to those that match the specified AS path.

restrict

Indicates that these routes are not to be considered as contributors of the specified aggregate. The specified protocol may be any of the protocols supported by gated.

Route filters

See ``Route filtering'' for a detailed explanation of how route filtering works. When no route filtering is specified (that is, when restrict is specified on the first line of a statement), all routes from the specified source will match that statement. If any filters are specified, only routes that match the specified filters will be considered as contributors. Put differently, if any filters are specified, an all restrict ; is assumed at the end of the list.

Files

References

RFC 827, RFC 891, RFC 904, RFC 1058, RFC 1105, RFC 1163, RFC 1164, RFC 1227, RFC 1245, RFC 1246, RFC 1253, RFC 1256, RFC 1265, RFC 1266, RFC 1267, RFC 1268, RFC 1269, RFC 1321, RFC 1370, RFC 1388, RFC 1397, RFC 1403, RFC 1583