Routers that can summarize routes with OSPF

In order to perform efficient routing of a large network by OSPF, it is important not only to divide the area but also to perform effective route summary along with the area division. In OSPF, the following two routers can be configured for route summary.

  • ABR
    Summarize inter-area routes for LSA Type 3 Network Summary LSAs
  • ASBR
    Summarize non-OSPF domain routes for LSA type 5 AS external LSAs

Route summary is not possible on routers other than ABR and ASBR.

Route summary for ABR

In multi-area OSPF, information about routes in other areas is advertised by LSA type 3 network summary LSAs. The ABR generates one LSA type 3 for each network address. Therefore, even if the OSPF network is divided into multiple areas, if route summary is not performed, the more network addresses in the other areas, the more LSA type 3 that will be advertised. And the size of the LSDB and routing table will increase. This makes it impossible to reduce the CPU process and memory usage of routers in the area even if the area is divided.

Figure When route summary is not performed
Figure When route summary is not performed

One solution to this situation is to use the Totally Stub area. The Totally Stubbed Area can be used to reduce the load on the router by advertising the default route in the area instead of the LSA type 3 for individual network addresses. Summarize all routes in other areas into a default route.

However, in a network configuration with multiple ABRs, traffic between areas may not always take the best route if it is totally stubbed and a default route is used. For example, in the following figure, we consider routing packets from R1 to 192.168.16.0/24. When two ABRs, ABR1 and ABR2, are configured for a Totally Stub Area, ABR1 and ABR2 advertise a default route to Area 1 instead of individual routes in other areas. Then R1 receives multiple default routes generated by ABR1 and ABR2, but it compares the metrics and selects the best route. R1 judges the default route received from ABR1 to be the best route and routes all packets destined for other networks in the area to ABR1. As a result, packets from R1 to 192.168.16.0/24 will be forwarded through ABR1 as shown in the blue line in the figure, and may not go through the best route.

Figure Example of a non-optimal path
Figure Example of a non-optimal path

In order to reduce the amount of LSA type 3 flowing in an area and to route packets between areas appropriately even in a network diagram with multiple ABRs, route summary is performed in the ABR.

Route summary offers the following advantages

  • Reduce the number of LSA Type 3 that ABR advertises in the area
  • Reduce the size of LSDB and routing table in the area
  • Unaffected by individual networks down in other areas
Figure Advantages of route summary
Figure Advantages of route summary

However, in order to perform route summary, IP addressing needs to be properly considered. For effective route summary, network addresses within an area need to be assigned from contiguous blocks of hierarchical IP addressing.

Configuring Route Summary for ABR

For ABR, to summarize inter-area routes, use the following command in OSPF configuration mode.

Configuring Route Summary for ABR

(config)#router ospf <process-id>
(config-router)#area <area-id> range <network-address> <subnetmask>

<process-id> : Process ID
<area-id> : area ID
<network-address> <subnetmask> : Network address/subnet mask of the summary route

Route summary of inter-area routes can be verified with the show ip ospf command.

Related article

The following article is an example of configuring route summary for ABR.

N-Study
Cisco OSPF Route Summary Configuration Example | N-Study
https://www.n-study.com/en/ospf-detail/cisco-ospf-route-summary-configuration-example/
This is an example of configuring OSPF inter-area route summary on a Cisco router. Notice the default cost of the default summary route.

Route summary for ASBR

The same can be said for LSA type 5 AS external LSAs generated by the ASBR. Network address routes in the non-OSPF domain are advertised by LSA type 5 to the OSPF domain by redistributing them to OSPF in ASBR. One LSA type 5 flows for one non-OSPF domain network address. Therefore, the greater the number of networks in the non-OSPF domain, the greater the number of LSA Type 5s flowing throughout the OSPF domain, and the more bloated the LSDB and routing table will become.

Figure When route summary is not performed(external routes)
Figure When route summary is not performed(external routes)

In order to build a more stable network by reducing the number of LSA type 5s flowing in the OSPF domain and reducing the size of the LSDB and routing table in the OSPF domain, we can use ASBR to summarize external routes. In other words, performing external route summary with ASBR has the following advantages.

  • Less LSA type 5 for ASBRs to advertise
  • The size of LSDB and routing table in the OSPF domain becomes smaller.
  • Unaffected by downed individual networks in non-OSPF domains
Figure Advantages of route summary(external routes)
Figure Advantages of route summary(external routes)

Configuring Route Summary for ASBR

In order to summarize external routes in ASBR, use the following command in OSPF configuration mode.

Configuring Route Summary for ASBR

(config)#router ospf <process-id>
(config-router)#summary-address <network-address> <subnetmask>

<process-id> : process ID
<networ-address> <subnetmask> : Network address/subnet mask of the summary route

You can verify the summarization of external routes by using the show ip ospf summary-address command.

How the OSPF works