OSPF external route summarization

External routes in OSPF are carried by type 5 LSA (external LSAs). Type 5 LSAs are generated by ASBR (Autonomous System Boundary Router), that’s why external route summarization must be configured on ASBR. Stub area do not allow ASBR because they don’t allow type 5 LSAs.

To configure OSPF external route summarization access ASBR and under OSPF configuration mode type following command:

 summary-address ip-address mask [not-advertise] [tag tag]

where not-advertise is optional and means that range matched will not be advertised, tag value is used to control redistribution and can be used further by a route map.

See below the scenario used in this lab.

Next you see configurations on routers for OSPF to work, and then we ‘ll configure summarization.

R1

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#no shutdown 
R1(config-if)#interface fastethernet 0/1
R1(config-if)#ip address 192.168.0.1 255.255.255.0
R1(config-if)#no shutdown 

R1(config)#router ospf 1
R1(config-router)#network 192.168.0.0 0.0.0.255 area 0
R1(config-router)#network 192.168.1.0 0.0.0.255 area 1

R2

R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 192.168.0.2 255.255.255.0
R2(config-if)#no shutdown

R2(config)#router ospf 1
R2(config-router)#network 192.168.0.0 0.0.0.255 area 0

BORDER

BORDER(config)#interface fastEthernet 0/0
BORDER(config-if)#ip address 192.168.1.2 255.255.255.0
BORDER(config-if)#no shutdown 
BORDER(config-if)#interface loopback 0
BORDER(config-if)#ip address 192.168.2.1 255.255.255.0

BORDER(config)#ip route 10.0.0.0 255.255.255.0 loopback 0
BORDER(config)#ip route 10.0.1.0 255.255.255.0 loopback 0
BORDER(config)#ip route 10.0.2.0 255.255.255.0 loopback 0
BORDER(config)#ip route 10.0.3.0 255.255.255.0 loopback 0

BORDER(config)#router ospf 1
BORDER(config-router)#network 192.168.1.0 0.0.0.255 area 1
BORDER(config-router)#redistribute static subnets

In this scenario BORDER router will be ASBR, I simulated external interface by Loopback0 and few static routes which were redistributed in OSPF on BORDER.

Let’s see if external routes (redistirbuted routes) arrive to R2:

R2#show ip route 

... output ommited ...

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 4 subnets
O E2 10.0.2.0 [110/20] via 192.168.0.1, 00:00:16, FastEthernet0/0
O E2 10.0.3.0 [110/20] via 192.168.0.1, 00:00:16, FastEthernet0/0
O E2 10.0.0.0 [110/20] via 192.168.0.1, 00:00:11, FastEthernet0/0
O E2 10.0.1.0 [110/20] via 192.168.0.1, 00:00:16, FastEthernet0/0
C    192.168.0.0/24 is directly connected, FastEthernet0/0
O IA 192.168.1.0/24 [110/20] via 192.168.0.1, 00:18:35, FastEthernet0/0
R2#

As you can see, all external routes are present on R2. It’s time to summarize external routes, this is done on BORDER router with the command discussed earlier:

BORDER(config-router)#summary-address 10.0.0.0 255.255.252.0

Now we’ll check again routing table on R2:

R2#show ip route 

... output ommited ...

Gateway of last resort is not set

     10.0.0.0/22 is subnetted, 1 subnets
O E2 10.0.0.0 [110/20] via 192.168.0.1, 00:01:04, FastEthernet0/0
C    192.168.0.0/24 is directly connected, FastEthernet0/0
O IA 192.168.1.0/24 [110/20] via 192.168.0.1, 00:18:35, FastEthernet0/0
R2#

You can see that all external routes are replaced by only one external route. 17255 views