OSPF Route Summarization

ccnp-routing
Shows why OSPF allows summarization only between areas and how to configure area range on ABRs, with routing tables before and after.
Published

Dec 10, 2017

Configuration

We want to advertise 10.10.1.0/25 and 10.10.1.128/25 as a 10.10.1.0/24 to R2:

image
Let us see the routing table of R2 before summarization: 
R2#show ip route ospf
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
O IA     10.10.1.1/32 [110/3] via 172.16.1.4, 00:35:20, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:16:04, GigabitEthernet0/0
O IA     10.10.1.129/32 [110/3] via 172.16.1.4, 00:35:20, GigabitEthernet0/0
                        [110/3] via 172.16.1.3, 00:16:04, GigabitEthernet0/0
O IA     10.10.2.1/32 [110/3] via 172.16.1.4, 00:35:20, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:16:04, GigabitEthernet0/0
O IA     10.10.3.1/32 [110/3] via 172.16.1.4, 00:35:20, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:16:04, GigabitEthernet0/0
O IA     10.10.35.0/29 [110/2] via 172.16.1.3, 00:16:04, GigabitEthernet0/0
O IA     10.10.45.0/29 [110/2] via 172.16.1.4, 00:35:20, GigabitEthernet0/0
R3 
R3(config)#router ospf 1
R3(config-router)#area 0 range 10.10.1.0 255.255.255.0
Let us see the routing table of R2 after summarization on R3 
R2#show ip route ospf
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 7 subnets, 3 masks
O IA     10.10.1.0/24 [110/3] via 172.16.1.3, 00:00:05, GigabitEthernet0/0
O IA     10.10.1.1/32 [110/3] via 172.16.1.4, 00:36:11, GigabitEthernet0/0
O IA     10.10.1.129/32 [110/3] via 172.16.1.4, 00:36:11, GigabitEthernet0/0
O IA     10.10.2.1/32 [110/3] via 172.16.1.4, 00:36:11, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:16:55, GigabitEthernet0/0
O IA     10.10.3.1/32 [110/3] via 172.16.1.4, 00:36:11, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:16:55, GigabitEthernet0/0
O IA     10.10.35.0/29 [110/2] via 172.16.1.3, 00:16:55, GigabitEthernet0/0
O IA     10.10.45.0/29 [110/2] via 172.16.1.4, 00:36:11, GigabitEthernet0/0

Now let us do the summarization on R4 as well.

R4 
R4(config)#router ospf 1
R4(config-router)#area 0 range 10.10.1.0 255.255.255.0
Let us see the routing table of R2 after summarization on both R3 and R4: 
R2#show ip route ospf
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
O IA     10.10.1.0/24 [110/3] via 172.16.1.4, 00:10:28, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:11:50, GigabitEthernet0/0
O IA     10.10.2.1/32 [110/3] via 172.16.1.4, 00:47:56, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:28:40, GigabitEthernet0/0
O IA     10.10.3.1/32 [110/3] via 172.16.1.4, 00:47:56, GigabitEthernet0/0
                      [110/3] via 172.16.1.3, 00:28:40, GigabitEthernet0/0
O IA     10.10.35.0/29 [110/2] via 172.16.1.3, 00:28:40, GigabitEthernet0/0
O IA     10.10.45.0/29 [110/2] via 172.16.1.4, 00:47:56, GigabitEthernet0/0
  • Note:
    • On ABR We used area X range IP MASK under the router configuration
    • On ASBR we use summary-address IP MASK under the router configuration

One more thing.

Let us see the routing table of R3 which was one of the ABRs we did summarization on: 
R3#show ip route ospf
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 8 subnets, 3 masks
O        10.10.1.0/24 is a summary, 00:00:25, Null0
O        10.10.1.1/32 [110/2] via 10.10.35.5, 00:00:25, GigabitEthernet0/5
O        10.10.1.129/32 [110/2] via 10.10.35.5, 00:00:25, GigabitEthernet0/5
O        10.10.2.1/32 [110/2] via 10.10.35.5, 00:00:25, GigabitEthernet0/5
O        10.10.3.1/32 [110/2] via 10.10.35.5, 00:00:25, GigabitEthernet0/5
O        10.10.45.0/29 [110/2] via 10.10.35.5, 00:00:25, GigabitEthernet0/5

The ABR that creates the summary route will create a null0 interface to prevent loops (if a loop happened, it would continue until the IP TTL expires). We know that we have more specific routes in the routing tables for a couple of 10.10.x.x networks. However, when a packet is received destined to 10.10.1.0 but there is no destination, the packet is sent to the blackhole null0 interface.

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