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Route Filtering Using Prefix Lists

The general syntax for configuring a prefix list is as follows:

Router(config)#ip prefix-list list-name [seq seq-value] deny | permit
network/len [ge ge-value] [le le-value]

The table that follows describes the parameters for this command.




The name of the prefix list


(Optional) Applies a sequence number to the entry being created or deleted


(Optional) Specifies the sequence number


Denies access to matching conditions


Permits access for matching conditions


(Mandatory) The network number and length (in bits) of the netmask


(Optional) Applies ge-value to the range specified


(Optional) Specifies the lesser value of a range (the “from” portion of the range description)


(Optional) Applies le-value to the range specified


(Optional) Specifies the greater value of a range (the “to” portion of the range description)

A router tests for prefix list matches from the lowest sequence number to the highest.

By numbering your prefix-list statements, you can add new entries at any point in the list.

The following examples show how you can use the prefix-list command to filter networks using some of the more commonly used options.

Router(config)#ip prefix-list ROSE permit le 24

Creates a prefix list that will accept a netmask of up to 24 bits (le meaning less than or equal to) in routes with the prefix Because no sequence number is identified, the default number of 5 is applied.

Router(config)#ip prefix-list ROSE deny ge 25

Creates a prefix list that will deny routes with a netmask of 25 bits or greater (ge meaning greater than or equal to) in routes with the prefix Because no sequence number is identified, the number 10 is applied—an increment of 5 over the previous statement.

NOTE: This configuration will per-mit routes such as or, but will deny a more spe-cific subnet such as

Router(config)#ip prefix-list TOWER permit ge 16 le 24

Creates a prefix list that permits all prefixes in the address space that have a netmask of between 16 and 24 bits (greater than or equal to 16 bits, and less than or equal to 24 bits).

Router(config)#ip prefix-list TEST seq 5 permit

Creates a prefix list and assigns a sequence number of 5 to a statement which permits only the default route

Router(config)#ip prefix-list TEST seq 10 permit ge 30 le 30

Creates a prefix list and assigns a sequence number of 10 to a statement that permits any prefix with a netmask of exactly 30 bits.

Router(config)#ip prefix-list TEST seq 15 permit le 32

Creates a prefix list and assigns a sequence number of 15 to a statement that permits any address or subnet (permit any).

Router(config)#no ip prefix-list TEST seq 10

Removes sequence number 10 from the prefix list.

Configuration Example: Using a Distribute List That References a Prefix List to Control Redistribution

Figure 4-7 shows the network topology for the configuration that follows, which demonstrates how to control redistribution with a prefix list using the commands covered in this chapter. Assume that all basic configurations and EIGRP and OSPF routing have been configured correctly.

Figure 4-7

Figure 4-7 Network Topology for Distribute List Configuration with Prefix Lists

The objective is to prevent networks and from being redistributed into the OSPF domain.

HULL(config)#ip prefix-list FILTER seq 5 permit

Creates a prefix list called FILTER with a first sequence number of 5 that explicitly permits the network.

HULL (config)#ip prefix-list FILTER seq 10 permit

Adds a second line to the FILTER prefix list that explicitly permits the network.

HULL(config)#router ospf 1

Enters OSPF process ID 1 routing process.

HULL(config-router)#redistribute eigrp 10 subnets

Redistributes all EIGRP networks into OSPF.

HULL(config-router)#distribute-list prefix FILTER out eigrp 10

Creates an outbound distribute list to filter routes being redistributed from EIGRP into OSPF that references the prefix list.

NOTE: The implicit deny any statement at the end of the prefix list prevents routing updates about any other network from being advertised. As a result, networks and will not be redistributed into OSPF.

Verifying Prefix Lists

show ip prefix-list [detail | summary]

Displays information on all prefix lists. Specifying the detail keyword includes the description and the hit count (the number of times the entry matches a route) in the display.

clear ip prefix-list prefix-list-name [network/length]

Resets the hit count shown on prefix list entries.

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