Route Summarization in OSPF

By | October 30, 2013

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

Route summarization involves consolidating multiple routes into a single advertisement. Proper route summarization directly affects the bandwidth, memory and CPU, that are consumed by the OSPF process.

  • If a network link fails or flaps, the topology change will not be propagated into the backbone or other areas.
  • It protects routers from needless routing table recalculations.
  • Because the SPF calculation places a significant demand on the router’s CPU, proper summarization is an imperative part of OSPF configuration.

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.

Using Route Summarization

Area1 & 2Let’s assume the following networks in routing table:

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24

Prefix’s Binary Splitting
  8        16       24         32
192.168.0.0 11000000 10101000 0000 0000 000000000
192.168.1.0 11000000 10101000 0000 0001 000000000
192.168.2.0 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. 255.255.252.0 and summarized route is 192.168.0.0/22

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.