Cisco Systems 78-11741-02 manual Configuring BGP Route Filtering by Neighbor, IPC-304

Configuring BGP

Configuring Basic BGP Features

Configuring BGP Route Filtering by Neighbor

You can filter BGP advertisements in two ways:

•Use autonomous system path filters, as with the ip as-pathaccess-listglobal configuration command and the neighbor filter-listrouter configuration command

•Use access or prefix lists, as with the neighbor distribute-listrouter configuration command. Filtering using prefix lists is described in the “Configuring BGP Filtering Using Prefix Lists” section.

If you want to restrict the routing information that the Cisco IOS software learns or advertises, you can filter BGP routing updates to and from particular neighbors. You can either define an access list or a prefix list and apply it to the updates.

Note Distribute-list filters are applied to network numbers and not autonomous system paths.

To filter BGP routing updates, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# neighbor {ip-address	Filters BGP routing updates to and from neighbors as
peer-group-name}distribute-list{access-list-number	specified in an access list.
access-list-name} {in out}	Note The neighbor prefix-listrouter configuration
	command can be used as an alternative to the
	neighbor distribute-list router configuration
	command, but you cannot use both commands to
	configure the same BGP peer in any specific
	direction. These two commands are mutually
	exclusive, and only one command (neighbor
	prefix-list or neighbor distribute-list) an be applied
	for each inbound or outbound direction.

Note Although the neighbor prefix-listrouter configuration command can be used as an alternative to the neighbor distribute-listcommand, do not use attempt to apply both the neighbor prefix-listand neighbor distribute-listcommand filtering to the same neighbor in any given direction. These two commands are mutually exclusive, and only one command (neighbor prefix-listor neighbor distribute-list) can be applied for each inbound or outbound direction.

Configuring BGP Filtering Using Prefix Lists

Prefix lists can be used as an alternative to access lists in many BGP route filtering commands. The section “How the System Filters Traffic by Prefix List” describes the way prefix list filtering works. The advantages of using prefix lists are as follows:

•Significant performance improvement in loading and route lookup of large lists.

•Support for incremental updates. Filtering using extended access lists does not support incremental updates.

Cisco IOS IP Configuration Guide

IPC-304