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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
1.20 AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 C613-50032-01 REV D
BGP and BGP4+ Prefix Lists
Prefix lists can be used as an alternative to access lists in many BGP or BGP4+ route
filtering commands. The advantages of using IPv4 or IPv6 prefix lists are as follows:
■ More of a performance improvement in loading and route lookup of large lists.
■ Support for updates. Filtering using access lists does not support updates.
■ The command-line interface to use access lists to filter BGP updates is more difficult.
■ More flexibility
Before using a prefix list in a BGP or BGP4+ command, first set up the prefix list, and assign
sequence numbers to entries in the prefix list.
BGP and BGP4+ filtering by prefix lists involves matching the prefixes of IPv4 or IPv6 routes
with those listed in the prefix list. 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 of a prefix list.
The router begins the search at the top of the prefix list, with the sequence number 1.
Once a match or deny occurs, the router need not go through the rest of the prefix list.
For efficiency, add the most common matches or denies near the top of the list, using the
seq parameter in the ip prefix-list (IPv4 Prefix List) and ipv6 prefix-list (IPv6 Prefix
List) Global Configuration mode commands. The show ip prefix-list (IPv4 Prefix List)
command and the show ipv6 prefix-list (IPv6 Prefix List) command output includes
sequence numbers.
Sequence numbers are generated automatically unless you disable automatic generation.
If you disable the automatic generation of sequence numbers, you need to specify the
sequence number for each entry with the seq parameter of the ip prefix-list (IPv4 Prefix
List) and ipv6 prefix-list (IPv6 Prefix List) Global Configuration mode commands.