Cisco Systems 3750E Configuring BGP Community Filtering, Ip community-listcommunity-list-number

Chapter 38 Configuring IP Unicast Routing

Configuring BGP

sequence number command; to reenable automatic generation, use the ip prefix-list sequence number command. To clear the hit-count table of prefix list entries, use the clear ip prefix-listprivileged EXEC command.

Configuring BGP Community Filtering

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

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. AS administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is identified by the COMMUNITIES attribute, an optional, transitive, global attribute in the numerical range from

1to 4294967200. These are some predefined, well-known communities:

•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.

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 learning, advertising, or redistributing 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. As with 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 set the COMMUNITIES attribute and match clauses based on communities, see the match community-listand set community route-map configuration commands in the “Using Route Maps to Redistribute Routing Information” section on page 38-80.

By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES attribute be sent to the neighbor at an IP address by using the neighbor send-communityrouter configuration command.

Beginning in privileged EXEC mode, follow these steps to create and to apply a community list:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	ip community-listcommunity-list-number	Create a community list, and assign it a number.
	{permit deny} community-number	• The community-list-numberis an integer from 1 to 99 that
		identifies one or more permit or deny groups of communities.
		• The community-numberis the number configured by a set
		community route-map configuration command.
Step 3
	router bgp autonomous-system	Enter BGP router configuration mode.
Step 4
	neighbor {ip-address peer-group name}	Specify that the COMMUNITIES attribute be sent to the neighbor at
	send-community	this IP address.
		Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02			38-57