CCNP ROUTE: 16. OSPF route summarization - Manual Summarization at ASBRs

When redistributing the routes, the ASBR creates a Type 5 External LSA for each redistributed subnet, listing the subnet number as the LSID and listing the mask as one of the fields in the LSA.
Adding the summary-address prefix mask OSPF subcommand, OSPF will then attempt to summarize the external routes by creating a Type 5 LSA for the summary route, and by no longer advertising the Type 5 LSAs for the subordinate subnets.

Note: The summary-address command cannot explicitly set the metric of the summary route.

The rules for summarization are:
■ The ASBR compares the summary route’s range of addresses with all routes redistributed into OSPF on that ASBR to find any subordinate subnets (subnets that sit inside the summary route range).
  If at least one subordinate subnet exists, the ASBR advertises the summary route.
■ The ASBR does not advertise the subordinate subnets.
■ To create the summary, the ASBR actually creates a Type 5 LSA for the summary route.
■ The ASBR assigns the summary route the same metric as the lowest metric route amongst all subordinate subnets.
■ If no subordinate subnets exist, the ASBR does not advertise the summary.
■ Unlike the area range command, the summary-address command cannot be used to directly set the metric of the summary route.

General syntax:
summary-address {{ip-address mask} | {prefix mask}}

Configure redistribution from EIGRP into OSPF on R4, making R4 an ASBR:
R4(config)#router ospf 1
R4(config-router)#redistribute eigrp 1 subnets

Now, routes to networks in the EIGRP domain are propagated into OSPF. Checking R1 for "O E2" routes (OSPF external Type 2 routes):
R1#show ip route
...
O E2 192.168.44.0/24 [110/20] via 192.168.0.14, 00:00:25, Serial0/1
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
O E2    172.16.0.12/30 [110/20] via 192.168.0.14, 00:00:25, Serial0/1
O E2    172.16.0.8/30 [110/20] via 192.168.0.14, 00:00:25, Serial0/1
O E2    172.16.0.4/30 [110/20] via 192.168.0.14, 00:00:25, Serial0/1
O E2    172.16.0.0/30 [110/20] via 192.168.0.14, 00:00:26, Serial0/1
O E2    172.16.111.0/24 [110/20] via 192.168.0.14, 00:00:26, Serial0/1
O E2 192.168.55.0/24 [110/20] via 192.168.0.14, 00:00:26, Serial0/1

We can create a summary route on R4 to be advertised to the OSPF domain, instead of advertising all four 172.16.0.X networks.
The summary address would be: 172.16.0.0/28

R4(config)#router ospf 1
R4(config-router)#summary-address 172.16.0.0 255.255.255.240

The summary is created on ASBR R4:
R4#show ip route
...
     172.16.0.0/16 is variably subnetted, 6 subnets, 3 masks
C       172.16.0.12/30 is directly connected, Serial0/2
D       172.16.0.8/30 [90/2681856] via 172.16.0.1, 02:00:11, Serial0/0
D       172.16.0.4/30 [90/2195456] via 172.16.0.14, 02:00:14, Serial0/2
                      [90/2195456] via 172.16.0.1, 02:00:14, Serial0/0
C       172.16.0.0/30 is directly connected, Serial0/0
O       172.16.0.0/28 is a summary, 00:00:32, Null0

The summary is advertised into the OSPF domain:
R1#show ip route
...
     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E2    172.16.0.0/28 [110/20] via 192.168.0.14, 00:02:28, Serial0/1