Allied Telesis BGP4, VERSION 5.4.3-2.6 manual BGP Community Filtering

BGP and BGP4+ Introduction

BGP Community Filtering

BGP supports transit policies via controlled distribution of routing information. The distribution of routing information is based on one of the following three values:

■IP address.

■The value of the autonomous system path attribute.

■The value of the communities attribute (as described in this section).

The communities attribute is a way to group destinations into communities and apply routing decisions based on the communities. This method simplifies the configuration of a BGP speaker that controls distribution of routing information.

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities.

The communities attribute is an optional, transitive, global attribute in the numerical range from 1 to 4,294,967,200. Along with Internet community, there are a few predefined, well-known communities, as follows:

■internet—Advertise this route to the Internet community. All routers belong to it.

■no-export—Do not advertise this route to eBGP peers.

■no-advertise—Do not advertise this route to any peer (internal or external).

■local-as—Do not advertise this route to peers outside the local autonomous system. This route will not be advertised to other autonomous systems or sub-autonomous systems when confederations are configured.

Based on the community, you can control which routing information to accept, prefer, or distribute to other neighbors. When routes are aggregated, the resulting aggregate has a communities attribute that contains all communities from all the initial routes.

You can use community lists to create groups of communities to use in a match clause of a route map, and a series of community lists can be created. Statements are checked until a match is found. As soon as one statement is satisfied, the test is concluded.

To create a community list, use the following command in Global Configuration mode:

awplus(config)#

ip community-list <listname> This command creates a community-list. To set communities {denypermit} .<community> attributes and match clauses based on communities, see the

match community (Route Map) command and the set community (Route Map) command for further information.

By default, the communities attribute is sent to a neighbor. To specify that the communities attribute are not sent to the neighbor, use the command below in Router Configuration mode:

awplus(config-router)#

no neighbor <neighborid> This command specifies that the communities attribute be sent send-community {bothextended to the neighbor at this IP address. Both standard and extended

standard} communities can be specified with the both keyword. Only standard or only extended can be specified with the standard and extended keywords

	Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.34	AlliedWare PlusTM Operating System - Software Version 5.4.3-2.6	C613-50032-01 REV D