Appendix D IP Prefix Lists

Prefix lists became available in Cisco IOS Software Release 12.0(3)T. You can use prefix lists as a simpler alternative to standard IP access lists for routing advertisement filtering with routing protocols. Although prefix lists are most commonly put to use in Border Gateway Protocol (BGP) configurations, this appendix demonstrates other ways that you can use prefix lists to support other routing protocols such as Enhanced Interior Gateway Routing Protocol (EIGRP). Prefix lists introduce a more streamlined way to create filters for network prefix advertisements by following these rules:

• Like a ccess lists, prefix lists are processed sequentially from top to bottom. When a match is made, processing stops and the rest of the entries are not read.

• Entrie s can be added to the prefix lists at any time.

• An empty prefix lists permits all prefixes by default.

• Prefix lists do not use wildcard masks like access lists; they use a subnet length mask (for instance, /24).

• Unlike access lists, lines in prefix lists can be edited by the use of the sequence number.

• Prefix lists contain an implicit deny any at the end of each list.

• Sequence numbers are automatically generated; however, automatic sequence generation can be stopped.

Prefix lists are configured from global configuration mode using the following command:

ip prefix-listlist-name | list-number [sequencesequence-value]deny | permit network-address/length [gege-value] [lele-value]

Table D-1 shows the meaning for the prefix list syntax.

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