In Figure 1, to demonstrate the effect of route summarization, R1 is configured to summarize the internal area 1 routes.
To manually configure interarea route summarization on an ABR, use the area area-id range address mask router configuration mode command. This instructs the ABR to summarize routes for a specific area before injecting them into a different area, via the backbone as type 3 summary LSAs.
Note: In OSPFv3, the command is identical except for the IPv6 network address. The command syntax for OSPFv3 is area area-id range prefix/prefix-length.
Figure 2 summarizes the two internal area 1 routes into one OSPF interarea summary route on R1. The summarized route 10.1.0.0/22 actually summarizes four network addresses, 10.1.0.0/24 to 10.1.3.0/24.
Figure 3 displays the IPv4 routing table of R1. Notice how a new entry has appeared with a Null0 exit interface. The Cisco IOS automatically creates a bogus summary route to the Null0 interface when manual summarization is configured to prevent routing loops. A packet sent to a null interface is dropped.
For example, assume R1 received a packet destined for 10.1.0.10. Although it would match the R1 summary route, R1 does not have a valid route in area 1. Therefore, R1 would refer to the routing table for the next longest match, which would be the Null0 entry. The packet would get forwarded to the Null0 interface and dropped. This prevents the router from forwarding the packet to a default route and possibly creating a routing loop.
Figure 4 displays the updated R3 routing table. Notice how there is now only one interarea entry going to the summary route 10.1.0.0/22. Although this example only reduced the routing table by one entry, summarization could be implemented to summarize many networks. This would reduce the size of routing tables.
Use the Syntax Checker in Figure 5 to summarize the area 2 routes on R3.