OSPF: Summarization at ASBR

R1 is the ASBR, R1 is redistributing Static routes into OSPF networks.

Summarization has to be done at R1 to optimize the routing table.

Before summarization:

R2#sh ip route ospf
172.16.0.0/24 is subnetted, 4 subnets
O E2    172.16.0.0 [110/200] via 172.30.0.1, 00:00:01, FastEthernet0/0
O E2    172.16.1.0 [110/200] via 172.30.0.1, 00:00:06, FastEthernet0/0
O E2    172.16.2.0 [110/200] via 172.30.0.1, 00:00:06, FastEthernet0/0
O E2    172.16.3.0 [110/200] via 172.30.0.1, 00:00:06, FastEthernet0/0

172.30.0.0/24 is subnetted, 3 subnets

R3#sh ip route ospf
172.16.0.0/24 is subnetted, 4 subnets
O E2    172.16.0.0 [110/200] via 172.30.0.1, 00:00:30, FastEthernet0/0
O E2    172.16.1.0 [110/200] via 172.30.0.1, 00:00:35, FastEthernet0/0
O E2    172.16.2.0 [110/200] via 172.30.0.1, 00:00:35, FastEthernet0/0
O E2    172.16.3.0 [110/200] via 172.30.0.1, 00:00:35, FastEthernet0/0

172.30.0.0/24 is subnetted, 3 subnets

R5#sh ip route ospf
172.16.0.0/24 is subnetted, 4 subnets
O E2    172.16.0.0 [110/200] via 172.30.10.2, 00:01:19, Serial0/0
O E2    172.16.1.0 [110/200] via 172.30.10.2, 00:01:23, Serial0/0
O E2    172.16.2.0 [110/200] via 172.30.10.2, 00:01:23, Serial0/0
O E2    172.16.3.0 [110/200] via 172.30.10.2, 00:01:23, Serial0/0

172.30.0.0/24 is subnetted, 3 subnets

R4#sh ip route ospf
172.16.0.0/24 is subnetted, 4 subnets
O E2    172.16.0.0 [110/200] via 172.30.20.3, 00:01:42, Serial0/0
O E2    172.16.1.0 [110/200] via 172.30.20.3, 00:01:47, Serial0/0
O E2    172.16.2.0 [110/200] via 172.30.20.3, 00:01:47, Serial0/0
O E2    172.16.3.0 [110/200] via 172.30.20.3, 00:01:47, Serial0/0

172.30.0.0/24 is subnetted, 3 subnets

After summarization at ASBR R1:

R1(config)#router ospf 1
R1(config-router)#summary-address 172.16.0.0 255.255.252.0
R1(config-router)#end

R1#sh ip route

172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
S       172.16.0.0/24 is directly connected, Null0
O       172.16.0.0/22 is a summary, 00:00:34, Null0
S       172.16.1.0/24 is directly connected, Null0
S       172.16.2.0/24 is directly connected, Null0
S       172.16.3.0/24 is directly connected, Null0

R2#sh ip route ospf
172.16.0.0/22 is subnetted, 1 subnets
O E2    172.16.0.0 [110/200] via 172.30.0.1, 00:01:18, FastEthernet0/0
172.30.0.0/24 is subnetted, 3 subnets

R3#sh ip route ospf
172.16.0.0/22 is subnetted, 1 subnets
O E2    172.16.0.0 [110/200] via 172.30.0.1, 00:01:46, FastEthernet0/0
172.30.0.0/24 is subnetted, 3 subnets

R4#sh ip route ospf
172.16.0.0/22 is subnetted, 1 subnets
O E2    172.16.0.0 [110/200] via 172.30.20.3, 00:02:04, Serial0/0
172.30.0.0/24 is subnetted, 3 subnets

R5#sh ip route ospf
172.16.0.0/22 is subnetted, 1 subnets
O E2    172.16.0.0 [110/200] via 172.30.10.2, 00:02:30, Serial0/0
172.30.0.0/24 is subnetted, 3 subnets

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