Cisco IE 4000, Software Configuration Manual

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Configuring IP Unicast Routing
Configuring BGP
Configuring BGP Filtering by Neighbor
You can filter BGP advertisements by using AS-path filters, such as the as-path access-list global configuration
command and the neighbor filter-list router configuration command. You can also use access lists with the neighbor
distribute-list router configuration command. Distribute-list filters are applied to network numbers. See Controlling
Advertising and Processing in Routing Updates, page 920 for information about the distribute-list command.
You can use route maps on a per-neighbor basis to filter updates and to 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 inbound and outbound updates, matching is supported based on AS path, community, and network numbers.
Autonomous-system path matching requires the
match as-path access-lis t route-map command, community-based
matching requires the match community-list route-map command, and network-based matching requires the ip
access-list global configuration command.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing, page 850 .
DETAILED STEPS
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.
EXAMPLE
The following router configuration mode example applies list 39 to incoming advertisements from neighbor172.16.4.1.
List 39 permits the advertisement of network 10.109.0.0.
Switch(config)# router bgp 109
Switch(config-router)#
network 10.108.0.0
Switch(config-router)# neighbor 172.16.4.1 distribute-list 39 in
Configuring BGP Filtering By Access Lists
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
Using Regular Expressions in
BGP 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.
Command Purpose
1. configure terminal Enter global configuration mode.
2. router bgp autonomous-system Enable a BGP routing process, assign it an AS number, and
enter router configuration mode.
3. neighbor { ip-address | peer-group
name } distribute-list
{ access-list-number | name } { in | out }
(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.
4. neighbor { ip-address | peer-group
name } route-map map-tag { in | out }
(Optional) Apply a route map to filter an incoming or
outgoing route.
5. end Return to privileged EXEC mode.
6. show ip bgp neighbors Verify the configuration.
7. copy running-config startup-config (Optional) Save your entries in the configuration file.