CCNP ROUTE 2.0: OSPF Route Summarisation

OSPF Route Summarisation consolidates multiple routes into a single advertisement; thus reducing the number of LSA flooding and the size of the LSDB and routing tables, which also reduces CPU and Memory utilisation. There are 2 types of summarisation:

  • Inter-area route summarisation (O IA) – routes from OSPF.
  • External route summarisation (O E1 or O E2) – routes from other routing protocols redistributed into OSPF.

Route summarisation is performed on an ABR for inter-area routes and ASBR for external routes. The Type 1 and Type 2 LSAs generated inside each area are converted into Type 3 LSAs in other areas. An ABR summarises Type 3 LSAs and an ASBR summarises Type 5 LSAs

Without summarisation every time a route is removed from an area, an LSA would be sent to the DR which in turn would notify all routers, causing unnecessary network traffic and router overhead. By using route summarisation only the summarised route is propagated into the backbone area 0. If a network link fails the topology change is localised and not propagated to the backbone area, which increases stability of the network and reduces LSA flooding. Summarisation prevents the other routers from having to re-run the SPF algorithm.

Example of Summarisation

In the example below we will configure 4 routers, to demonstrate how to summarise Inter-area and External route summarisation. Router R2 will act as an ABR distributing the routes learned from R3 and also an ASBR, redistributing the EIGRP routes from R4. When summarisation is configured it will need to be configured on the ASBR and ABR.

Below is a screenshot of the routing table of router R1 with no summarisation configured on R1. We can see the 4 loopback networks learned from OSPF area 1 (O IA) and 4 the 4 loopback networks redistributed from EIGRP into OSPF (O E1).

Displaying the OSPF database reveals entries for each of the inter-area and external routes (output below abbreviated).

Configure Inter-Area Route Summarisation

Inter-area route summarisation is configured on the ABR. In this example it will convert the 4 routes learned from area 1 into a single /22 route. On the ABR the command “area x range address mask” is configured under the OSPF process.

R2 (config)# router ospf 1
R2 (config-router)# area 1 range

As you can see in the screenshot below the 4 inter-area (O IA) routes have been replaced with a single summarised route.

Configure External Route Summarisation

External route summarisation is configured on the ASBR (in this example is also R2). To configure OSPF route summarisation of redistributed routes from other routing protocols we use the command “summary-address address mask” under the OSPF process.

R2 (config)# router ospf 1
R2 (config-router)# summary-address

As reflected in the screenshot below you can clearly see that the external routes (O E1) have been replaced with a single summarised route.

Post configuration of summarisation, checking the OSPF database reflects only a single LSA entry for the /22 route for both the inter-area and external route.

show ip ospf database summary

show ip ospf database external

In a production network using OSPF route summarisation would help reduce the size of the LSDB, reduce CPU/Memory utilisation, help speed up convergence and increase stability of the network.

When a summary route is created a null 0 interface is created on the ABR and ASBR. The null 0 interface is used in the event the summarising router receives a packet for an unknown subnet that is part of a summarised route, the packet would match the summary route based on longest match. The packet would be forwarded to the null 0 interface and dropped. This is used as a loop prevention mechanism.

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