Cisco Systems 78-11741-02 Configuring Advanced BGP Features, Using Route Maps to Modify Updates

Configuring BGP

Configuring Advanced BGP Features

Configuring Advanced BGP Features

The tasks in this section are for configuring advanced BGP features.

Using Route Maps to Modify Updates

You can use a route map on a per-neighbor basis to filter updates and modify various attributes. A route map can be applied to either inbound or outbound updates. Only the routes that pass the route map are sent or accepted in updates.

On both the inbound and the outbound updates, we support matching based on autonomous system path, community, and network numbers. Autonomous system path matching requires the as-pathaccess-list global configuration command, community based matching requires the ip community-listglobal configuration command and network-based matching requires the ip access-listglobal configuration command. To apply a route map to incoming and outgoing routes, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# neighbor {ip-address	Applies a route map to incoming or outgoing routes.
peer-group-name}route-mapmap-name {in out}

See the “BGP Route Map Examples” section at the end of this chapter for BGP route map examples.

Resetting eBGP Connections Immediately upon Link Failure

Normally, when a link between external neighbors goes down, the BGP session will not be reset immediately.To reset the eBGP session as soon as an interface goes down, use the following command in router configuration mode:

Command	Purpose
Router(config-router)#bgp fast-external-fallover	Resets eBGP sessions automatically.

Configuring Aggregate Addresses

CIDR enables you to create aggregate routes (or supernets) to minimize the size of routing tables. You can configure aggregate routes in BGP either by redistributing an aggregate route into BGP or by using the BGP Conditional Aggregation feature. An aggregate address will be added to the BGP table if at least one more specific entry is in the BGP table.

Cisco IOS IP Configuration Guide

IPC-311