Cisco Systems 78-11741-02 manual Configuring BGP Conditional Advertisement, IPC-314

Configuring BGP

Configuring Advanced BGP Features

Specifying the Format for the Community

A BGP community is displayed in a two-part format 2 bytes long in the show ip bgp community EXEC command output, and wherever communities are displayed in the router configuration, such as router maps and community lists. In the most recent version of the RFC for BGP, a community is of the form AA:NN, where the first part is the autonomous system number and the second part is a 2-byte number. The Cisco default community format is in the format NNAA.

To display BGP communities in the new format, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip bgp-community new-format	Displays and parses BGP communities in the format
	AA:NN.

Configuring BGP Conditional Advertisement

BGP advertises routes from its routing table to external peers (peers in different autonomous systems) by default. The BGP Conditional Advertisement feature provides additional control of route advertisement depending on the existence of other prefixes in the BGP table. Normally, routes are propagated regardless of the existence of a different path. The BGP Conditional Advertisement feature uses the non-exist-map and the advertise-map to track routes by the route prefix. If a route prefix is not present in the non-exist-map, the route specified by the advertise-map is announced. The announced route is installed to the BGP routing table as a locally originated route and will behave as a locally originated route. The announced route will be originated by BGP only if the corresponding route exists in the BGP table. After the prefix is locally originated by BGP, BGP will advertise the prefix to internal and external peers. If the route prefix is present, the route in the advertise-map is not announced.

Conditional advertisement can be useful in a multihomed network, in which some prefixes are to be advertised to one of the providers, only if information from the other provider is missing. This condition would indicate a failure in the peering session, or partial reachability.

If the same information is advertised to all providers in a multihomed environment, the information is duplicated in the global BGP table. When the BGP Conditional Advertisement feature is used, only partial routes are advertised to each provider, and the size of the global BGP table is not increased with redundant information. The administrator can also guarantee the path that inbound traffic will follow because only specific paths are advertised to providers.

Note The conditional BGP announcements are sent in addition to the normal announcements that a BGP router sends to its peers.

Note Autonomous system path list information cannot be used for conditional advertisement because the IP routing table does not contain autonomous system path information.

Cisco IOS IP Configuration Guide

IPC-314