Prefix Lists

Jan 15, 2012

If you choose not to enter a sequence number manually with the prefix-list command the sequence numbers are automatically generated in increments of 5

Prefix lists work by matching the prefixes in the list to the prefixes of routes that are under scrutiny.

The manor in which they work is similar to ACLs, where there is a match the route is used or discarded

Follows the rules;

  • If the route is permitted the route is used

  • If a route is denied the route is not used

  • At the bottom of every prefix list is an implicit deny any.

  • When multiple entries of a prefix list matches a given prefix the entry with the smallest seq number (ie the 1st in the list)

  • The router begins the search at the top of the prefix list, when a match is made the search stops. Processing time will be reduced if the most common matches are at the top.

  • A sequence number does not need to be specified when removing a configuration entry

      ip prefix-list <name> [seq <number>] deny|permit <network/len> [ge <value>] [le <value>]

Rob Edwards

Northern (UK) chap focusing on platforms, automation, cloud and cloud native applications. Recovering network engineer, although it turns out networking is as important, if not more, now than before!

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