Cisco Systems 78-11741-02 manual Configuring BGP Path Filtering by Neighbor, IPC-308

Configuring BGP

Configuring Basic BGP Features

Router#	show	ip prefix-listprefix-list-name	Displays all entries of a prefix list that are more specific than
[network/length] longer			the given network and length.
Router#	show	ip prefix-listprefix-list-name	Displays the entry of a prefix list that matches the given
[network/length] first-match			prefix (network and length of prefix).

Clearing the Hit Count Table of Prefix List Entries

To clear the hit count table of prefix list entries, use the following command in EXEC mode:

Command	Purpose
Router# clear ip prefix-list prefix-list-name	Clears the hit count table of the prefix list entries.
[network/length]

Configuring BGP Path Filtering by Neighbor

In addition to filtering routing updates based on network numbers, you can specify an access list filter on both incoming and outbound updates based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. To specify the access list filter, define an autonomous system path access list and apply it to updates to and from particular neighbors. See the “Regular Expressions” appendix in the Cisco IOS Terminal Services Configuration Guide for more information on forming regular expressions.

To configure BGP path filtering, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1
	Router# ip as-path access-list access-list-number	Defines a BGP-related access list.
	{permit deny} as-regexp
Step 2
	Router# router bgp as-number	Enters router configuration mode.
Step 3
	Router(config-router)# neighbor {ip-address	Establishes a BGP filter.
	peer-group-name}filter-listaccess-list-number {in
	out}

See the “BGP Path Filtering by Neighbor Examples” section at the end of this chapter for an example of BGP path filtering by neighbor.

Disabling Next Hop Processing on BGP Updates

You can configure the Cisco IOS software to disable next hop processing for BGP updates to a neighbor. Disabling next hop processing might be useful in nonmeshed networks such as Frame Relay or X.25, where BGP neighbors might not have direct access to all other neighbors on the same IP subnet. There are two ways to disable next hop processing:

•Provide a specific address to be used instead of the next hop address (manually configuring each address).

•Use a route map to specify that the address of the remote peer for matching inbound routes, or the local router for matching outbound routes (automatic method).

Cisco IOS IP Configuration Guide

IPC-308