Configuring BGP
Configuring Advanced BGP Features
IPC-314
Cisco IOS IP Configuration Guide

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:
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-m ap, the rou te specifie d by the adver tise-map i s 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.
Command Purpose
Router(config)# ip bgp-community new-format Displays and parses BGP communities in the format
AA:NN.