Cisco Systems 78-11741-02 manual Disabling Automatic Summarization of Network Numbers, IPC-312

Configuring BGP

Configuring Advanced BGP Features

To create an aggregate address in the routing table, use the following commands in router configuration mode:

Command	Purpose
Router(config-router)# aggregate-addressaddress	Creates an aggregate entry in the BGP routing table.
mask
Router(config-router)# aggregate-addressaddress	Generates autonomous system set path information.
mask as-set
Router(config-router)# aggregate-address	Advertises summary addresses only.
address-mask summary-only
Router(config-router)# aggregate-addressaddress	Suppresses selected, more specific routes.
mask suppress-mapmap-name
Router(config-router)# aggregate-addressaddress	Generates an aggregate based on conditions specified by the
mask advertise-map map-name	route map.
Router(config-router)# aggregate-addressaddress	Generates an aggregate with attributes specified in the route
mask attribute-map map-name	map.

See the “BGP Aggregate Route Examples” section at the end of this chapter for examples of using BGP aggregate routes.

Disabling Automatic Summarization of Network Numbers

In BGP Version 3, when a subnet is redistributed from an IGP into BGP, only the network route is injected into the BGP table. By default, this automatic summarization is enabled. To disable automatic network number summarization, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# no auto-summary	Disables automatic network summarization.

Configuring BGP Community Filtering

BGP supports transit policies via controlled distribution of routing information. The distribution of routing information is based on one of the following three values:

•IP address (see the “Configuring BGP Route Filtering by Neighbor” section earlier in this chapter).

•The value of the autonomous system path attribute (see the “Configuring BGP Path Filtering by Neighbor” section earlier in this chapter).

•The value of the communities attribute (as described in this section).

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

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. Autonomous system administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is carried as the communities attribute.

Cisco IOS IP Configuration Guide

IPC-312