Cisco 3560G 48TS Software Configuration Guide - Page 662

Configuring BGP Chapter 30 Configuring IP Unicast Routing Step 6 Step 7 Command show ip bgp neighbors copy running-config startup-config Purpose Verify the configuration. (Optional) Save your entries in the configuration file. Use the no neighbor distribute-list command to remove the access list from the neighbor. Use the no neighbor route-map map-tag router configuration command to remove the route map from the neighbor. Another method of filtering is to 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. (Refer to the "Regular Expressions" appendix in the Cisco IOS Dial Services Command Reference for more information on forming regular expressions.) To use this method, define an autonomous system path access list, and apply it to updates to and from particular neighbors. Beginning in privileged EXEC mode, follow these steps to configure BGP path filtering: Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Command configure terminal ip as-path access-list access-list-number {permit | deny} as-regular-expressions router bgp autonomous-system neighbor {ip-address | peer-group name} filter-list {access-list-number | name} {in | out | weight weight} end show ip bgp neighbors [paths regular-expression] copy running-config startup-config Purpose Enter global configuration mode. Define a BGP-related access list. Enter BGP router configuration mode. Establish a BGP filter based on an access list. Return to privileged EXEC mode. Verify the configuration. (Optional) Save your entries in the configuration file. Configuring Prefix Lists for BGP Filtering You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including the neighbor distribute-list router configuration command. The advantages of using prefix lists include performance improvements in loading and lookup of large lists, incremental update support, easier CLI configuration, and greater flexibility. Filtering by a prefix list involves matching the prefixes of routes with those listed in the prefix list, as when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or denied is based upon these rules: • An empty prefix list permits all prefixes. • An implicit deny is assumed if a given prefix does not match any entries in a prefix list. • When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry identifies the entry with the lowest sequence number. By default, sequence numbers are generated automatically and incremented in units of five. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry. You can specify sequence values in any increment. If you specify increments of one, you cannot insert additional entries into the list; if you choose very large increments, you might run out of values. 30-54 Catalyst 3560 Switch Software Configuration Guide 78-16156-01