12.2.4 Configuring an Area Border Router

Points to note

You can use the area range command with the not-advertise parameter specified to prevent the networks in a masked address range from being advertised outside the area.

For a given area, you can specify more than one address range for route summarization and for route advertisement suppression. A router or network in the area can be assigned an address not included in any of the address ranges you specify. However, when building a network, by assigning addresses to suit the topology first and then configuring summarization for address ranges in a manner appropriate for the topology, you can efficiently reduce the amount of OSPF routing traffic without impairing the ability of the protocol to select the best routes.

Command examples

The following is an example of configuring route summarization for an area border router associated with areas 0 and 1.

1. (config)# router ospf 1

   (config-router)# area 0 range 10.0.0.0 255.255.254.0

   Having learned routes in the range of network 10.0.0.0 masked by 255.255.254.0 in area 0, the router advertises a summarized route to area 1.

2. (config-router)# area 1 range 10.0.2.0 255.255.255.0

   Having learned routes in the range of network 10.0.2.0 masked by 255.255.255.0 in area 1, the router advertises a summarized route to area 0.

3. (config-router)# network 10.0.0.0 0.0.0.255 area 0

   Interfaces in the network 10.0.0.0/24 range belong to area 0.

4. (config-router)# network 10.0.2.0 0.0.0.255 area 1

   Associates interfaces in the 10.0.2.0/24 network range with area 1.