Prefix Lists

Whether a prefix is permitted or denied is based upon the following rules:

• If a route is permitted, the route is used.

• If the prefix list is empty, all prefixes are permitted.

• At the bottom of every prefix list is an implicit deny all. Thus, if given prefix does not match any entries of a prefix list, it is denied.

• When multiple entries of a prefix list match a given prefix, the entry with the smallest sequence number is used.

• The router begins the search at the top of the prefix list, with the sequence number 1. When a match is made, the search stops. Processing time will be reduced if the most common matches or denies are near the top of the list. This will prevent having to process criteria that are seldom met every time a route is examined.

• Sequence numbers are generated by default. To configure the sequence numbers manually, use the seq seq-value argument of the ip prefix-list command.

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

Table 9-10 Displaying Prefix Lists—Command Options

Command Description show ip prefix-list [detail | summary] Displays information about all prefix lists, including the hit count, which is the number of times that a match has been found for the criteria in the prefix list. This is very important in troubleshooting for capacity planning and security.

show ip prefix-list [detail | summary] name Displays a table showing the entries in a prefix list identified by name.

show ip prefix-list name [network/len] Displays the filtering associated with the node based on the absolute of the defined prefix.

show ip prefix-list name [seq seq-num] Displays the prefix list entry with a given sequence number.

show ip prefix-list name [network/len] Displays all entries of a prefix list that are more longer specific than the given network and length.

show ip prefix-list name [network/len] Displays the entry of a prefix list that matches the first-match given prefix (network and length of prefix).

NOTE The show commands always include the sequence numbers in their output.

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