OSPF Route Summarization Commands
| Type of router |
Router OSPF subcommand |
|---|---|
| ASBR | summary-address <network-id> <prefix-mask> [not-advertise] |
| ABR | area <area-id> range <network-id> <prefix-mask> [advertise |not-advertise] |
Note: Advertise and not-advertise keywords are optional. The not-advertise keyword is used to filter the subnets implied by the summary.
Configuring OSPF Route Summarization in Cisco
Pre-requisites: Open Shortest Path First (OSPF) protocol States and Configuring Multi-Area OSPF in Cisco
Route summarization in any Interior Gateway Protocol (IGP) creates a single route entry with a prefix & prefix length larger than one or smaller component routes. For example summary route 10.1.0.0/16 includes subnets 10.1.1.0/24,10.1.4.132/30 and other subnets in the range of 10.1.0.0 to 10.1.255.255. Summary routes reduce the size of routing tables and topology databases, indirectly improving convergence. All routers in the same OSPF area must have:
- Identical Link State Databases after flooding is complete.
- Same summary routes.
- Must be missing the same component subnets of each summary.
To make the above thing happen, OSPF allows route summarization only if the routes are injected into an area by the following types of the router:
- Area Border Router (ABR) –inter-area routes
- Autonomous System Boundary Router (ASBR) -external routes