Cisco 3750G Configuration Guide - Page 877

Chapter 35 Configuring IP Unicast Routing Configuring BGP 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-list and set community route-map configuration commands in the "Using Route Maps to Redistribute Routing Information" section on page 35-79. 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-community router 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-list community-list-number Create a community list, and assign it a number. {permit | deny} community-number • The community-list-number is an integer from 1 to 99 that identifies one or more permit or deny groups of communities. • The community-number is 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. Step 5 set comm-list list-num delete (Optional) Remove communities from the community attribute of an inbound or outbound update that match a standard or extended community list specified by a route map. Step 6 exit Return to global configuration mode. Step 7 ip bgp-community new-format (Optional) Display and parse BGP communities in the format AA:NN. A BGP community is displayed in a two-part format 2 bytes long. The Cisco default community format is in the format NNAA. In the most recent RFC for BGP, a community takes the form AA:NN, where the first part is the AS number and the second part is a 2-byte number. Step 8 end Return to privileged EXEC mode. Step 9 show ip bgp community Verify the configuration. Step 10 copy running-config startup-config (Optional) Save your entries in the configuration file. Configuring BGP Neighbors and Peer Groups Often many BGP neighbors are configured with the same update policies (that is, the same outbound route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update policies can be grouped into peer groups to simplify configuration and to make updating more efficient. When you have configured many peers, we recommend this approach. OL-8550-02 Catalyst 3750 Switch Software Configuration Guide 35-57