We will use below topology:
R4 has two inter-area routes to 1.1.1.1:
R4#sho ip route 1.1.1.1
Routing entry for 1.1.1.1/32
Known via "ospf 1", distance 110, metric 4, type inter area
Last update from 192.168.45.5 on Serial1/0, 00:00:11 ago
Routing Descriptor Blocks:
192.168.45.5, from 5.5.5.5, 00:00:11 ago, via Serial1/0
Route metric is 4, traffic share count is 1
* 192.168.34.3, from 2.2.2.2, 00:00:11 ago, via Serial1/1
Route metric is 4, traffic share count is 1
If we want to filter the path from R2 through 192.168.34.3 we could do it in below way:
R4(config)#access-list 1 permit 1.1.1.1
R4(config)#access-list 2 permit 2.2.2.2
R4(config)#route-map OSPF deny 10
R4(config-route-map)#match ip address 1
R4(config-route-map)#match ip route-source 2
R4(config-route-map)#route-map OSPF permit 20
R4(config-route-map)#router ospf 1
R4(config-router)#distribute-list route-map OSPF in
R4(config-router)#^Z
R4#sho ip route 1.1.1.1
Routing entry for 1.1.1.1/32
Known via "ospf 1", distance 110, metric 4, type inter area
Last update from 192.168.45.5 on Serial1/0, 00:00:12 ago
Routing Descriptor Blocks:
* 192.168.45.5, from 5.5.5.5, 00:00:12 ago, via Serial1/0
Route metric is 4, traffic share count is 1
But let’s say we have a task that asks us to create a new area attached to R4 as follows:-
Now we need two virtual-links and look at what happened to our route 1.1.1.1.
R4(config)#router ospf 1
R4(config-router)#area 1 virtual-link 5.5.5.5
R4(config-router)#area 1 virtual-link 2.2.2.2
*Mar 3 01:31:13.935: %OSPF-5-ADJCHG: Process 1, Nbr 5.5.5.5 on
OSPF_VL2 from LOADING to FULL, Loading Done
*Mar 3 01:31:16.979: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on
OSPF_VL3 from LOADING to FULL, Loading Done
R4#sho ip route 1.1.1.1
Routing entry for 1.1.1.1/32
Known via "ospf 1", distance 110, metric 4, type intra area
Last update from 192.168.34.3 on Serial1/1, 00:00:00 ago
Routing Descriptor Blocks:
* 192.168.45.5, from 1.1.1.1, 00:00:00 ago, via Serial1/0
Route metric is 4, traffic share count is 1
192.168.34.3, from 1.1.1.1, 00:00:00 ago, via Serial1/1
Route metric is 4, traffic share count is 1
Well now we are learning 1.1.1.1 as an inter-area route. So the router-ID advertising the LSA has changed. We are now learning the route from type-1 LSAs originated by R1 directly in Area 0.
If we filter based on router-id then we will lose both paths. So now we need to filter based on next-hop:
R4(config)#access-list 3 permit 192.168.34.3
R4(config)#no route-map OSPF
R4(config)#route-map OSPF deny 10
R4(config-route-map)#match ip add 1
R4(config-route-map)#match ip next-hop 3
R4(config-route-map)#route-map OSPF pe 20
R4(config-route-map)#^Z
R4#sho ip route 1.1.1.1
Routing entry for 1.1.1.1/32
Known via "ospf 1", distance 110, metric 4, type intra area
Last update from 192.168.45.5 on Serial1/0, 00:00:02 ago
Routing Descriptor Blocks:
* 192.168.45.5, from 1.1.1.1, 00:00:02 ago, via Serial1/0
Route metric is 4, traffic share count is 1
Thanks for reading the blog.