Cisco Systems 78-11741-02 How the System Filters Traffic by Prefix List, Creating a Prefix List

Configuring BGP

Configuring Basic BGP Features

•More user-friendly command-line interface (CLI). The command-line interface for using access lists to filter BGP updates is difficult to understand and use because it uses the packet filtering format.

•Greater flexibility

Before using a prefix list in a command, you must set up a prefix list, and you may want to assign

sequence numbers to the entries in the prefix list.

How the System Filters Traffic by Prefix List

Filtering by prefix list involves matching the prefixes of routes with those listed in the prefix list. When there is a match, the route is used. More specifically, whether a prefix is permitted or denied is based upon the following 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.

•When multiple entries of a prefix list match a given prefix, the longest, most specific match is chosen.

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, you may want to put the most common matches or denies near the top of the list, using the seq argument in the ip prefix-listglobal configuration command. The show commands always include the sequence numbers in their output.

Sequence numbers are generated automatically unless you disable this automatic generation. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry using the sequence-valueargument of the ip prefix-listglobal configuration command.

Regardless of whether the default sequence numbers are used in configuring a prefix list, a sequence number need not be specified when removing a configuration entry.

show commands include the sequence numbers in their output.

Creating a Prefix List

To create a prefix list, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# ip prefix-listlist-name[seq	Creates a prefix list with the name specified for the list-name
sequence-value] {deny permit network/length} [ge	argument.
ge-value] [le le-value]

Note To create a prefix list you must enter at least one permit or deny clause.

To remove a prefix list and all of its entries, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# no ip prefix-listlist-name	Removes a prefix list with the name specified for list-name.
[seq sequence-value] {deny permit network/length}
[ge ge-value] [le le-value]
	Cisco IOS IP Configuration Guide

IPC-305