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OSPF Inter-area route summarization



OSPF is link state routing protocol that works on the concept of areas. All areas must have same LSDB (link state database); hence OSPF summarization can only done on the border routers i.e. on ABR (Area border router) and ASBR (Autonomous system boundary router).In this document we discussed about route summarization between the areas.


Summarization between areas can be done on ABR by using single command under OSPF process:

area [area-id] range [ ip-address ] [mask ] [advertise | not-advertise |cost {cost}]

A) area-id= Identifier of the area about which routes are to be summarized

B) [ ip-address ] [mask]= Summary route to be advertise in areas

Summary route calculation:

Only 3 steps are required to calculate summary route

     1) Convert the addresses to binary format
     2) Locate the bit where the common pattern of digits ends
     3) The number of common bits is subnet mask and the summary route number is represented by the first IP address in the block.

Let’s assume the following networks in routing table:


Binary Splitting


  8        16       24         32

11000000 10101000 0000 0000 000000000

11000000 10101000 0000 0001 000000000

11000000 10101000 0000 0010 000000000


From the above table it shows first 22 bits are common, so summary route subnet mask is /22 i.e. and summarized route is

C) advertise = By default type 3 LSA is advertise
D) not-advertise =Its optional and used to filter type 3 LSA component of the networks specify in range command.
E) Cost =It’s optional When we have multiple ABRs we can Advertise summary route with cost for route manipulation. 

Configuration overview:

In following example router ABR1 is directly connected to R1 in area 1 and to R2 in backbone area0.

Topology Diagram:




On R1 loopbacks are created loopbacks for generating network prefixes and advertise them into OSPF as follow:

R1#sh ip interface brief | include Lo

Loopback0             YES manual up                    up

Loopback1             YES manual up                    up

Loopback2             YES manual up                    up

R2 in area 0 learns Type3 LSA for loopbacks created on R1 via ABR1 and installed into routing table as shown in below output:


R2#sh ip ospf database | Beg Summary

                Summary Net Link States (Area 0)


Link ID         ADV Router      Age         Seq#       Checksum        260         0x80000001 0x00B463        102         0x80000001 0x00B8FD        102         0x80000001 0x00AD08        102         0x80000001 0x00A212


R2#sh ip ospf database summary


            OSPF Router with ID ( (Process ID 100)


                Summary Net Link States (Area 0)


  Routing Bit Set on this LSA

  LS age: 7

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(Network)

  Link State ID: (summary Network Number)

  Advertising Router:

  LS Seq Number: 80000002

  Checksum: 0x5C62

  Length: 28

  Network Mask: /24

        TOS: 0  Metric: 2  


R2#sh ip route | Begin Gateway

Gateway of last resort is not set is subnetted, 2 subnets

O IA [110/2] via, 00:00:36, FastEthernet0/0

C is directly connected, FastEthernet0/0

O IA [110/3] via, 00:00:36, FastEthernet0/0

O IA [110/3] via, 00:00:36, FastEthernet0/0

O IA [110/3] via, 00:00:36, FastEthernet0/0

Configuring ABR1 to summarized loopback subnets available on R1 i.e.


ABR1(config)#router ospf 100

ABR1(config-router)#area 1 range




After configuring summarization on ABR1, R2 receives Type 3 i.e. not individual type 3 LSA for each loopback address from ABR1 shown below:


R2#sh ip route | Begin Gateway

Gateway of last resort is not set is subnetted, 2 subnets

O IA [110/2] via, 00:16:36, FastEthernet0/0

C is directly connected, FastEthernet0/0

O IA [110/3] via, 00:06:00, FastEthernet0/0

Discard route:

It's interesting to look at these summary routes on the ABR, which is responsible for doing all of the summarization. The ABR also includes the summary routes, but they aren't real routes, so it simply points them to its Null0 interface called discard routes. They help to prevent routing loops during summarization:

ABR1#sh ip route ospf

O [110/2] via, 00:15:01, FastEthernet0/0

O [110/2] via, 00:15:01, FastEthernet0/0

O [110/2] via, 00:15:01, FastEthernet0/0

O is a summary, 00:15:01, Null0


Discard route is generated by default, and you don't need to create it. If you want to disable creation of the discard route, you can use the no discard-route command as follows:

ABR1(config)#router ospf 100

ABR1(config-router)#discard-route internal



Benefits of OSPF Summarization:

1) Reduce the number of routing table entries.

2) Reduce Type3 flooding and saves router CPU resource.

3) Summarization can also be used for route manipulation by using longest match prefix concept

Related Information:

OSPF Design Guide

OSPF  Vesion 2

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