Configuring OSPF Virtual Links w/ GRE tunnels

In this blogtorial we are going to discuss on how to link an OSPF area which is not directly connected to Area 0 using GRE tunnels. Why would you ever want to do this? I don't know :) maybe in the CCIE lab exam. I strongly recommend not using GRE or Virtual links but they do exists for a reason. 


We are going to be building on the topology from my previous blogtorials. So let's get started.

Relevant configurations are posted below but basically all we are doing is creating a GRE tunnel between R5 and R4 and putting the tunnel in Area 0.




R7#sh ip route

Gateway of last resort is not set

     172.16.0.0/24 is subnetted, 1 subnets

C       172.16.7.0 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 1 subnets

C       10.3.3.0 is directly connected, 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)

Now let's get R5 and R4 configured and see the magic in action. 

R5#sh run int tunnel 0

interface Tunnel0

 ip address 192.168.1.2 255.255.255.0 ** Drop an ip address on the tunnel interface **

 tunnel source 10.1.1.5 ** Specify the tunnel source which is facing towards R4 **

 tunnel destination 10.1.1.4 ** Specify the tunnel destination which is R5 **

end

R5#sh run | sec ospf

router ospf 1

 log-adjacency-changes

 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

 network 192.168.1.0 0.0.0.255 area 0 ** Add the tunnel interface to Area 0 ** 

Pretty much the same config on R4 except a few changes. 

R4#sh run int tunnel 0

interface Tunnel0

 ip address 192.168.1.1 255.255.255.0

 tunnel source 10.1.1.4

 tunnel destination 10.1.1.5

end

R4#sh run | sec ospf

router ospf 1

 log-adjacency-changes

 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

 network 192.168.1.0 0.0.0.255 area 0

Now let's take a look at R7 and witness the magic!!

R7#sh ip route

     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, 00:03:27, FastEthernet1/0

O E2    172.16.4.0/24 [110/20] via 10.3.3.5, 00:03:27, FastEthernet1/0

O E2    172.16.6.0/24 [110/20] via 10.3.3.5, 00:03:27, 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, 00:03:27, FastEthernet1/0

O E2    172.16.2.0/24 [110/20] via 10.3.3.5, 00:03:27, FastEthernet1/0

O E2    172.16.3.0/24 [110/20] via 10.3.3.5, 00:03:27, 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, 00:03:27, FastEthernet1/0

O IA    10.1.1.0 [110/2] via 10.3.3.5, 00:03:27, FastEthernet1/0

O IA    10.0.0.0 [110/1002] via 10.3.3.5, 00:03:27, FastEthernet1/0

O IA 192.168.1.0/24 [110/1001] via 10.3.3.5, 00:03:28, 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 100 percent (5/5), round-trip min/avg/max = 52/65/76 ms

As you can see a router in Area 2 (R7) can successfully ping the loopback on R6. 

More articles to come so stay tuned!! If you like my posts please subscribe :)