Configuring prefix lists

Configure a prefix list to allow or deny specific route updates. A prefix list policy specifies route prefixes to match. After a match occurs, the system uses the route.

The prefix list contains a set of contiguous or noncontiguous routes. Reference prefix lists by name from within a routing policy.

About this task

Important

Important

When you configure a prefix list for a route policy, add the prefix as a.b.c.d/32. You must enter the full 32-bit mask to exact a full match of a specific IP address.

You configure prefix lists on a VRF instance the same way you configure the GlobalRouter, except that you must use VRF Router Configuration mode.

Procedure

  1. Enter Global Configuration mode:

    enable

    configure terminal

  2. Configure a prefix list:

    ip prefix-list WORD<1-64> {A.B.C.D/X} [ge <0–32>] [le <0–32>]

  3. Optional: Rename an existing prefix list:

    ip prefix-list WORD<1-64> name WORD<1-64>

  4. Display the prefix list:

    show ip prefix-list [prefix {A.B.C.D}] [vrf WORD<0-16>] [vrfids WORD<0-512>] [WORD <1-64>]

Example

Configure a prefix-list. Display the prefix list.

Switch:1>enable
Switch:1#configure terminal
Switch:1(config)#ip prefix-list LIST1 47.17.121.50/255.255.255.0
Switch:1(config)#show ip prefix-list LIST1
================================================================================
                           Prefix List - GlobalRouter
================================================================================

        PREFIX           MASKLEN FROM TO
--------------------------------------------------------------------------------

List 1    LIST1:
        47.17.121.50     24      24   24
1 Total Prefix List entries configured
--------------------------------------------------------------------------------
Name Appendix for Lists Converted from Old Config:
@A=conv addr list, @N=conv net list, @NR=conv net list modified as range


Variable definitions

Use the data in the following table to use the ip prefix-list command.

Variable

Value

{A.B.C.D/X}

Specifies the IP address and the mask in one of the following formats:
  • a.b.c.d/x

  • a.b.c.d/x.x.x.x

  • default

ge <0–32>

Specifies the minimum length to match.

Lower bound and higher bound mask lengths together can define a range of networks.

le <0–32>

Specifies the maximum length to match.

Lower bound and higher bound mask lengths together can define a range of networks.

name WORD<1-64>

Renames the specified prefix list. The name length is 1–64 characters.

WORD<1-64>

Specifies the name for a new prefix list.

Use the data in the following table to use the show ip prefix-list command.

Variable

Value

{A.B.C.D}

Specifies the prefix to include in the command output.

vrf WORD<0-16>

Specifies the name of the VRF.

vrfids WORD<0-512>

Specifies the ID of the VRF and is an integer in the range of 0–512.

WORD<1-64>

Specifies a prefix list, by name, to use for the command output.

Use the following table to use the show ip prefix-list command output.

Variable

Value

PREFIX

Indicates the member of a specific prefix list.

MASKLEN

Indicates the prefix mask length in bits.

FROM

Indicates the prefix mask starting point in bits.

TO

Indicates the prefix mask endpoint in bits.