Cisco Systems 78-11741-02 manual IPC-313

Configuring BGP

Configuring Advanced BGP Features

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. A BGP speaker can set, append, or modify the community of a route when you learn, advertise, or redistribute routes. 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. Just like an access list, 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:

Command	Purpose
Router(config)# ip community-list	Creates a community list.
community-list-number {permit deny}
community-number

To set the communities attribute and match clauses based on communities, see the match community-listand set community route map configuration commands in the “Redistribute Routing Information” section in the “Configuring IP Routing Protocol-Independent Features” chapter.

By default, no communities attribute is sent to a neighbor. To specify that the communities attribute to be sent to the neighbor at an IP address, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# neighbor {ip-address	Specifies that the communities attribute be sent to the
peer-group-name}send-community [both standard	neighbor at this IP address. Both standard and extended
extended]	communities can be specified with the both keyword. Only
	standard or only extended can be specified with the standard
	and extended keywords.

To remove communities from the community attribute of an inbound or outbound update using a route map to filter and determine the communities to be deleted, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# set comm-list	Removes communities in a community attribute that match a
community-list-number delete	standard or extended community list.

Cisco IOS IP Configuration Guide

IPC-313