CCNP ROUTE: 15. OSPF route summarization - Manual Summarization at ABRs

OSPF allows summarization at both ABRs and ASBRs but not on other OSPF routers. The main reason is that the LSDB must be the same for all routers in a single area.

General syntax:
area area-id range ip-address mask [cost cost]

The rules for summarization are:
■ The configured area number refers to the area where the subnets exist; the summary will be advertised into all other areas connected to the ABR.
■ The ABR compares the summary route’s range of addresses with all intra-area OSPF routes, in the origin area, for which the ABR is creating Type 3 LSAs.
  If at least one subordinate subnet exists (subnets that sit inside the range), then the ABR advertises the summary route as a Type 3 LSA.
■ The ABR does not advertise the subordinate subnet’s Type 3 LSAs.
■ The ABR assigns a metric for the summary route’s Type 3 LSA, by default, to match the BEST metric (lowest cost) among all subordinate subnets.
■ The area range command can also explicitly set the cost of the summary.
■ If no subordinate subnets exist, the ABR does not advertise the summary.

We have to create a manual summary route for the following networks in Area 0 and advertise it to areas 2 and 3 (routers in areas 2 and 3 should not know about the links between routers in Area 0):
    192.168.0.0/29
192.168.0.12/30
    192.168.0.16/30
    192.168.0.20/30

The summary must be created on the ABR between the 3 areas: R9. The summary address would be: 192.168.0.0/27

Initially, R10 (Area 2) and R12 (Area 3) both have routes for each 192.168.0.X network in Area 0:
R9#show ip route
...
     192.168.0.0/24 is variably subnetted, 4 subnets, 2 masks
O       192.168.0.12/30 [110/128] via 192.168.0.21, 01:24:02, Serial0/0
O       192.168.0.0/29 [110/74] via 192.168.0.21, 01:24:02, Serial0/0
O       192.168.0.16/30 [110/128] via 192.168.0.21, 01:24:02, Serial0/0
C       192.168.0.20/30 is directly connected, Serial0/0

R10#show ip route
...
     192.168.0.0/24 is variably subnetted, 4 subnets, 2 masks
O IA    192.168.0.12/30 [110/192] via 10.0.1.1, 01:06:40, Serial0/0.1
O IA    192.168.0.0/29 [110/138] via 10.0.1.1, 01:06:40, Serial0/0.1
O IA    192.168.0.16/30 [110/192] via 10.0.1.1, 01:06:40, Serial0/0.1
O IA    192.168.0.20/30 [110/128] via 10.0.1.1, 01:06:40, Serial0/0.1

R12#show ip route
...
     192.168.0.0/24 is variably subnetted, 4 subnets, 2 masks
O IA    192.168.0.12/30 [110/138] via 192.168.3.1, 01:20:34, FastEthernet0/0
O IA    192.168.0.0/29 [110/84] via 192.168.3.1, 01:20:34, FastEthernet0/0
O IA    192.168.0.16/30 [110/138] via 192.168.3.1, 01:20:34, FastEthernet0/0
O IA    192.168.0.20/30 [110/74] via 192.168.3.1, 01:20:35, FastEthernet0/0

Creating the summary route on the ABR:
R9(config)#router ospf 1
R9(config-router)#area 0 range 192.168.0.0 255.255.255.224

Now, R9 created a summary (with an exit interface of Null0):
R9#show ip route
...
     192.168.0.0/24 is variably subnetted, 5 subnets, 3 masks
O       192.168.0.12/30 [110/128] via 192.168.0.21, 00:01:14, Serial0/0
O       192.168.0.0/29 [110/74] via 192.168.0.21, 00:01:14, Serial0/0
O       192.168.0.0/27 is a summary, 00:01:14, Null0
O       192.168.0.16/30 [110/128] via 192.168.0.21, 00:01:14, Serial0/0
C       192.168.0.20/30 is directly connected, Serial0/0

R10 and R12 have only the summary route advertised by R9:

R10#show ip route
...
     192.168.0.0/27 is subnetted, 1 subnets
O IA    192.168.0.0 [110/128] via 10.0.1.1, 00:02:16, Serial0/0.1

R12#show ip route
...
     192.168.0.0/27 is subnetted, 1 subnets
O IA    192.168.0.0 [110/74] via 192.168.3.1, 00:03:46, FastEthernet0/0

Also, notice that R10 and R12 are using the best metrics from the former specific routes (R10: 128 and R12: 74).