Cisco Systems 78-11741-02 manual Configuring a Prefix List Entry, To the entry, IPC-306

Configuring BGP

Configuring Basic BGP Features

Configuring a Prefix List Entry

You can add entries to a prefix list individually. To configure an entry in a prefix list, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# ip prefix-listlist-name[seq	Creates an entry in a prefix list and assigns a sequence number
sequence-value] {deny permit network/length} [ge	to the entry.
ge-value] [le le-value]

The optional ge and le keywords can be used to specify the range of the prefix length to be matched for prefixes that are more specific than the network/length argument. An exact match is assumed when neither ge nor le is specified. The range is assumed to be from ge-valueto 32 if only the ge attribute is specified, and from len to le-valueif only the le attribute is specified.

A specified ge-valueor le-valuemust satisfy the following condition:

len < ge-value <= le-value <= 32

For example, to deny all prefixes matching /24 in 128.0.0.0/8, use the following command:

ip prefix-list abc deny 128.0.0.0/8 ge 24 le 24

Note You can specify sequence values for prefix list entries in any increments you want (the automatically generated numbers are incremented in units of 5). If you specify the sequence values in increments of 1, you cannot insert additional entries into the prefix list. If you choose very large increments, you could run out of sequence values.

Configuring How Sequence Numbers of Prefix List Entries Are Specified

By default, the sequence numbers are automatically generated when you create a prefix list entry. Sequence numbers can be suppressed with the no ip prefix-listsequence-numberglobal configuration command. Sequence values are generated in increments of 5. The first sequence value generated in a prefix list would be 5, then 10, then 15, and so on. If you specify a value for an entry and then do not specify values for subsequent entries, the assigned (generated) sequence values are incremented in units of five. For example, if you specify that the first entry in the prefix list has a sequence value of 3, and then do not specify sequence values for the other entries, the automatically generated numbers will be 8, 13, 18, and so on.

To disable the automatic generation of sequence numbers, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# no ip prefix-list	Disables the automatic generation of the sequence numbers
sequence-number	for prefix list entries.

To re-enable automatic generation of the sequence numbers of prefix list entries, use the ip prefix-list sequence number command in router configuration mode:

Cisco IOS IP Configuration Guide

IPC-306