Configuring Prefix Lists For Bgp Filtering - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 31 Configuring IP Unicast Routing
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 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:
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.
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:
•
•
•
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.
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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.
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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring BGP
31-55