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).
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