OSPF(ASBR) Summary address

There are 2 summarizations in OSPF routing protocols: Area summarization and External summarization. The area summarization is the summarization between the area. We can use "area x range x.x.x.x x.x.x.x" format to do so.  However, in this post, we will talk about external summarization. We use "summary-address x.x.x.x x.x.x.x" format to achieve this. 

In the example below, we consider R1 and the ASBR of the OSPF because it is a gateway between routing protocols (RIP and OSPF). We will show the effect of the "summary-address x.x.x.x x.x.x.x" command here.

We have network 20.1.1.0/24 being redistributed from RIP to OSPF. The 20.1.1.0/24 network will be an external route and shown in the R3 routing table as E2 type. Below shown the result without using the summary address command and with the command enable.

Without "summary-address" command.

Lets view routing table of R3

R3
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     20.0.0.0/24 is subnetted, 1 subnets
O E2    20.1.1.0 [110/20] via 192.168.2.1, 00:00:01, FastEthernet0/0
     172.16.0.0/24 is subnetted, 3 subnets
C       172.16.1.0 is directly connected, Loopback0
C       172.16.2.0 is directly connected, Loopback1
C       172.16.3.0 is directly connected, Loopback2
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Loopback4
O E2 192.168.1.0/24 [110/20] via 192.168.2.1, 00:08:12, FastEthernet0/0
C    192.168.2.0/24 is directly connected, FastEthernet0/0

When enable the ASBR summarization at router R1

Let's see the configuration of R1 first regarding the external summarization.

R1

router ospf 1

 router-id 1.1.1.1

 log-adjacency-changes

 redistribute rip subnets

 network 172.16.0.0 0.0.255.255 area 0

 network 192.168.2.0 0.0.0.255 area 0

 summary-address 20.0.0.0 255.0.0.0

 redistribute ospf 1 metric 3

Let's now check the routing table of R3

R3

R3#sh ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

O E2 20.0.0.0/8 [110/20] via 192.168.2.1, 00:07:18, FastEthernet0/0

     172.16.0.0/24 is subnetted, 3 subnets

C       172.16.1.0 is directly connected, Loopback0

C       172.16.2.0 is directly connected, Loopback1

C       172.16.3.0 is directly connected, Loopback2

     10.0.0.0/24 is subnetted, 1 subnets

C       10.1.1.0 is directly connected, Loopback4

O E2 192.168.1.0/24 [110/20] via 192.168.2.1, 00:08:03, FastEthernet0/0

C    192.168.2.0/24 is directly connected, FastEthernet0/0