Configuring Prefix Lists For Bgp Filtering - Cisco 3032 Software Configuration Manual

Configuring BGP
Command
Step 6
show ip bgp neighbors
Step 7 copy running-config startup-config
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 filtering method 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 autonomous-system-path
filtering:
Command
Step 1
configure terminal
Step 2
ip as-path access-list access-list-number
{permit | deny} as-regular-expressions
Step 3 router bgp autonomous-system
Step 4 neighbor {ip-address | peer-group name}
filter-list {access-list-number | name} {in |
out | weight weight}
Step 5 end
Step 6 show ip bgp neighbors [paths
regular-expression]
Step 7 copy running-config startup-config

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.
Prefix-list filtering matches 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:
•
•
•
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An empty prefix list permits all prefixes.
An implicit deny is assumed if a prefix does not match any entries in a prefix list.
When multiple entries of a prefix list match a prefix, the sequence number of a prefix-list entry
identifies the entry with the lowest sequence number.
Purpose
Verify the configuration.
(Optional) Save your entries in the configuration file.
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.
Chapter 38 Configuring IP Unicast Routing
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