Cisco 3845 Software Guide - Page 618

Configuring BGP Chapter 29 Configuring IP Unicast Routing Step 3 Step 4 Step 5 Step 6 Step 7 Command neighbor {ip-address | peer-group name} distribute-list {access-list-number | name} {in | out} neighbor {ip-address | peer-group name} route-map map-tag {in | out} end show ip bgp neighbors copy running-config startup-config Purpose (Optional) Filter BGP routing updates to or from neighbors as specified in an access list. Note You can also use the neighbor prefix-list router configuration command to filter updates, but you cannot use both commands to configure the same BGP peer. (Optional) Apply a route map to filter an incoming or outgoing route. Return to privileged EXEC mode. 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. (See the "Regular Expressions" appendix in the Cisco IOS Dial Technologies Command Reference, Release 12.2 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. 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. 29-52 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01