Configuring Advanced OSPF - Virtual Links

In this blogtorial we are going to talk about OSPF virtual links. As you already know all areas must connect to area 0. However, if you have an area not connected to area 0 it is possible to use virtua links to accomplish this.


We are going to be building on the topology from my previous blogtorial "Configuring Advanced OSPF". So let's get started.

Relevant configurations are posted below.



R7 (in area 2) output before virtual links are configured.

R7#sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.1.1.5          1   FULL/BDR        00:00:34    10.3.3.5        FastEthernet1/0

R7# ping 172.16.6.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.6.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

R5 is now configured with Area 2 connecting to R7. We need to create a virtual link between R5 and R4 so Area 2 can be connected to Area 0.


R5#sh run | sec ospf
router ospf 1
 log-adjacency-changes
 area 1 virtual-link 10.0.0.4 ** area # (transit area # meaning which area are we going to use to link Area 2 and Area 0 which in this case happens to be Area 1) <router id of the Area 0 router> **
 network 10.1.1.0 0.0.0.255 area 1
 network 10.3.3.0 0.0.0.255 area 2
 network 172.16.5.0 0.0.0.255 area 1

R4#sh run | sec ospf

router ospf 1
 log-adjacency-changes
 area 1 virtual-link 10.1.1.5 ** transit area 1 and router-id of R5 **
 redistribute connected subnets
 network 10.0.0.0 0.0.0.255 area 0
 network 10.1.1.0 0.0.0.255 area 1

Notice R4 and R4 OSPF virtual link state = FULL.

R4#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

10.1.1.5          0   FULL/  -           -        10.1.1.5        OSPF_VL1

10.0.0.1          1   FULL/DR         00:00:35    10.0.0.1        FastEthernet1/0

10.0.0.2          1   FULL/DROTHER    00:00:36    10.0.0.2        FastEthernet1/0

172.16.3.1        1   FULL/DROTHER    00:00:36    10.0.0.3        FastEthernet1/0

10.1.1.5          1   FULL/DR         00:00:38    10.1.1.5        FastEthernet1/1

R5#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

10.0.0.4          0   FULL/  -           -        10.1.1.4        OSPF_VL0

10.0.0.4          1   FULL/BDR        00:00:38    10.1.1.4        FastEthernet1/0

172.16.7.1        1   FULL/DR         00:00:36    10.3.3.7        FastEthernet1/1

R7#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

10.1.1.5          1   FULL/BDR        00:00:37    10.3.3.5        FastEthernet1/0

R7#sh ip route

Gateway of last resort is not set

     172.16.0.0/16 is variably subnetted, 7 subnets, 2 masks

O IA    172.16.5.1/32 [110/2] via 10.3.3.5, 00:10:25, FastEthernet1/0

O E2    172.16.4.0/24 [110/20] via 10.3.3.5, 00:10:25, FastEthernet1/0

O E2    172.16.6.0/24 [110/20] via 10.3.3.5, 00:10:01, FastEthernet1/0

C       172.16.7.0/24 is directly connected, Loopback0

O E2    172.16.1.0/24 [110/20] via 10.3.3.5, 00:10:01, FastEthernet1/0

O E2    172.16.2.0/24 [110/20] via 10.3.3.5, 00:10:01, FastEthernet1/0

O E2    172.16.3.0/24 [110/20] via 10.3.3.5, 00:10:01, FastEthernet1/0

     10.0.0.0/24 is subnetted, 4 subnets

C       10.3.3.0 is directly connected, FastEthernet1/0

O E2    10.2.2.0 [110/20] via 10.3.3.5, 00:10:01, FastEthernet1/0

O IA    10.1.1.0 [110/2] via 10.3.3.5, 00:10:25, FastEthernet1/0

O IA    10.0.0.0 [110/3] via 10.3.3.5, 00:10:06, FastEthernet1/0

After the virtual links are configured notice the ping on R7 to the loopback on R6 (in the eigrp 10).

R7# ping 172.16.6.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.6.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 68/82/100 ms

What is next? Maybe virtual links using GRE tunnels, route filtering using route-maps/distribute-list/prefix-list. 

More articles to come so stay tuned!!