To create a prefix list or add a prefix list entry, use the ip prefix-list command in Global Configuration mode. Prefix lists allow matching of route prefixes with those specified in the prefix list. Each prefix list includes of a sequence of prefix list entries ordered by their sequence numbers. A router sequentially examines each prefix list entry to determine if the route‘s prefix matches that of the entry. An empty or nonexistent prefix list permits all prefixes. An implicit deny is assume if a given prefix does not match any entries of a prefix list. Once a match or deny occurs the router does not go through the rest of the list. A prefix list may be used within a route map to match a route‘s prefix using the match ip address command (see match ip address).
Up to 128 prefix lists may be configured. The maximum number of statements allowed in prefix list is 64.
| Default | No prefix lists are configured by default. When neither the ge nor the le option is configured, the destination prefix must match the network/length exactly. If the ge option is configured without the le option, any prefix with a network mask greater than or equal to the ge value is considered a match. Similarly, if the le option is configured without the ge option, a prefix with a network mask less than or equal to the le value is considered a match. |
| Format | ip prefix-list list-name {[seq number] {permit | deny} network/length [ge length] [le length] | renumber renumber-interval first-statement-number} |
| Mode | Global Configuration |
| Parameter | Description |
|---|---|
| list-name | The text name of the prefix list. Up to 32 characters. |
| seq number | (Optional) The sequence number for this prefix list statement. Prefix list statements are ordered from lowest sequence number to highest and applied in that order. If you do not specify a sequence number, the system will automatically select a sequence number five larger than the last sequence number in the list. Two statements may not be configured with the same sequence number. The value ranges from 1 to 4,294,967,294. |
| permit | Permit routes whose destination prefix matches the statement. |
| deny | Deny routes whose destination prefix matches the statement. |
| network/length | Specifies the match criteria for routes being compared to the prefix list statement. The network can be any valid IP prefix. The length is any IPv4 prefix length from 0 to 32. |
| ge length | (Optional) If this option is configured, then a prefix is only considered a match if its network mask length is greater than or equal to this value. This value must be longer than the network length and less than or equal to 32. |
| le length | (Optional) If this option is configured, then a prefix is only considered a match if its network mask length is less than or equal to this value. This value must be longer than the ge length and less than or equal to 32. |
| renumber | (Optional) Provides the option to renumber the sequence numbers of the IP prefix list statements with a given interval starting from a particular sequence number. The valid range for renumber-interval is 1–100, and the valid range for first-statement-number is 1–1000. |
The following example configures a prefix list that allows routes with one of two specific destination prefixes, 172.20.0.0/16 and 192.168.1.0/24:
(Extreme 220) (Routing)(config)# ip prefix-list apple seq 10 permit 172.20.0.0/16 (Extreme 220) (Routing)(config)# ip prefix-list apple seq 20 permit 192.168.10/24
The following example disallows only the default route.
(Extreme 220) (Routing)(config)# ip prefix-list orange deny 0.0.0.0/0 (Extreme 220) (Routing)(config)# ip prefix-list orange permit 0.0.0.0/0 ge 1
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