OSPF - Summarization

OSPF summarization can be configured on 2 types of routers:

ABR – Internal OSPF routes

ABSR – External OSPF routes

My topology:

R1 --- FR --- R2

|

R4 --- FR --- R3

Example 1 – Summarizing External routes

I have created several loopback interfaces on R4 (ABSR),

lo0: 4.4.0.4/24

lo1: 4.4.1.4/24

lo2: 4.4.2.4/24

lo3: 4.4.3.4/24

Now I will redistribute the directly connected loopback interfaces addresses to OSPF:

R4(config-if)#access-list 4 permit 4.4.0.0 0.0.3.255

R4(config)#route-map REDIS permit 10

R4(config-route-map)#match ip addr 4

R4(config-route-map)#router ospf 1

R4(config-router)#redistribute connected subnets route-map REDIS

on R3, we can see the redistributed routes as external routes:

To summarize the external routes, use the “summary-address” on R4

R4(config-router)#summary-address 4.4.0.0 255.255.252.0

You can see those redistributed external routes are summarized:

Example 2 – Summarizing Internal Routes

In this example I will try to summarize internal routes from area 1 on ABR (R2)

R1

lo0 – 1.1.0.1/24 (area 1)
lo1 – 1.1.1.1/24 (area 1)
lo2 – 1.1.2.1/24 (area 1)
lo3 – 1.1.3.1/24 (area 1)
s1/0.12 – area 1

R2
lo0 – area 0
s1/0.21 – area 1
fa0/0 – area 0

To summarize internal routes, use the “area X range” command:

R2(config-router)#area 1 range 1.1.0.0 255.255.252.0

on R3, you can see the summarized route 1.1.0.0/22:

Example 3 – Discard-route

The discard route is created automatically when you configure a summary route.

On R2 (ABR where Internal route summarization takes place):

O 1.1.0.0/22 is a summary, 00:08:54, Null0

On R4 (ASBR where External route summarization takes place)

O 4.4.0.0/22 is a summary, 00:15:05, Null0

The discard routes are created to stop forwarding loop.

Use the “no discard-route” command to remove the discard-route:

R2(config-router)#no discard-route internal
R4(config-router)#no discard-route external

On R2 and R4, the discard route is gone.

R2

R4