IP Prefix List

Use IP prefix lists with ge and le matching and route maps to filter routes and change OSPF external route types in a worked redistribution example.

• Like an ACL for network prefixes
• Processes sequentially
• For example:
  • ip prefix-list PL1 seq 5 permit 10.1.1.0/24
  • ip prefix-list PL1 seq 10 deny 0.0.0.0/0 le 32
• ge means minimum prefix length
• le means maximum prefix length
• IP prefix list can be used to filter LSAs. We will see this later
• IP prefix list can be applied using route maps

Route Map

• A set of sequentially processed rules that can match certain criteria and set attributes.
• Example:

  R1(config)#route-map RM1 permit 10
  R1(config-route-map)#match ip address prefix-list PL1
  R1(config-route-map)#set metric-type type-1

Configuration Example

See the example on the Redistribution into OSPF page.

R1#show ip route ospf
Gateway of last resort is not set

O E2  1.0.0.0/8 [110/20] via 10.10.1.2, 00:01:51, GigabitEthernet0/0

We are asked to advertise the network 1.1.0.0 which has a subnet mask of minimum /24 and maximum /30 as E1 as opposed to E2 using a route-map

ASBR(config)#ip prefix-list PL1 seq 10 permit 1.0.0.0/8 ge 24 le 30
ASBR(config)#route-map CONN->OSPF permit
ASBR(config-route-map)#match ip address prefix-list PL1
ASBR(config-route-map)#set metric-type 2
ASBR(config-route-map)#exit
ASBR(config)#route-map CONN->OSPF permit 20
ASBR(config-route-map)#exit
ASBR(config)#router ospf 1
ASBR(config-router)#redistribute connected subnets route-map CONN->OSPF
ASBR(config-router)#no summary-address 1.0.0.0 255.0.0.0

I issued no summary-address 1.0.0.0 255.0.0.0 to override the changes of the last example

R1#show ip route ospf
Gateway of last resort is not set

      1.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
O E1     1.1.1.0/30 [110/21] via 10.10.1.2, 00:00:09, GigabitEthernet0/0
O E1     1.1.2.0/24 [110/21] via 10.10.1.2, 00:00:09, GigabitEthernet0/0
O E2     1.3.0.0/16 [110/20] via 10.10.1.2, 00:00:09, GigabitEthernet0/0