In the previous article I’ve discuss about Router ID Concept.  And in this article I’ll discuss about how to configure OSPF Router ID on Cisco.

The Following is the topology are used :

Based on the topology above, we want to configure step by step to find out the router-id changes.

Configure interface G0/0 and G0/1 on each routers first, Then Configure OSPF. We’ll Configure Loopback interface later.

Configure IP Address R1 :

R1(config)#int g0/0
R1(config-if)#ip add 12.12.12.1 255.255.255.0
R1(config-if)#no shut
R1(config-if)#exit
R1(config)#int g0/1
R1(config-if)#ip add 192.168.100.1 255.255.255.0
R1(config-if)#no shut
R1(config-if)#exit

Configure IP Address R2 :

R2(config)#int g0/0
R2(config-if)#ip add 12.12.12.2 255.255.255.0
R2(config-if)#no shut
R2(config-if)#exit
R2(config)#int g0/1
R2(config-if)#ip add 192.168.200.1 255.255.255.0 
R2(config-if)#no shut
R2(config-if)#exit
R2(config)#

Configure OSPF on R1 :

R1(config)#router ospf 1
R1(config-router)#network 12.12.12.0 0.0.0.255 area 0
R1(config-router)#network 192.168.100.0 0.0.0.255 area 0
R1(config-router)#exit

Configure OSPF on R2 :

R2(config)#router ospf 1
R2(config-router)#network 12.12.12.0 0.0.0.255 area 0
R2(config-router)#network 192.168.200.0 0.0.0.255 area 0
R2(config-router)#exit

After adjacency between R1 and R2 are successfull, then show the neighbor to see the router id

R1 OSPF neighbor. As you can see the neighbor on R1 bellow, R1 detect the Neighbor of R2 with N-ID 192.168.200.1. Address 12.12.12.2 is ip address R2 interface that directly connect to R1 and 192.168.200.1 is also R2 ip address on active interface with highest value.

R1#show ip ospf neighbor

Neighbor ID      Pri    State       Dead Time     Address       Interface
192.168.200.1      1    FULL/BDR    00:00:35      12.12.12.2    GigabitEthernet0/0
R1#

R2 OSPF neigbor. Also on R2, you can see the neighbor of R1

R2#show ip ospf neighbor

Neighbor ID     Pri    State     Dead Time     Address      Interface
192.168.100.1     1    FULL/DR   00:00:39      12.12.12.1   GigabitEthernet0/0
R2#

Show the interface brief on R1 & R2 :

R1#show ip int brief
Interface             IP-Address    OK? Method  Status  Protocol
GigabitEthernet0/0    12.12.12.1    YES manual  up      up 
GigabitEthernet0/1    192.168.100.1 YES manual  up      up 
R1#

R2#show ip int brief
Interface            IP-Address    OK? Method  Status Protocol
GigabitEthernet0/0   12.12.12.2    YES manual  up     up 
GigabitEthernet0/1   192.168.200.1 YES manual  up     up 
R2#

Look at the interface brief above, both routers that use the highest active ip address because those routers area not configure the static router-id.

Now we want to test to add loopback interface with highest ip address as you can see to topology for each routers.

Configure Loopback ip on R1:

R1(config)#int lo0
R1(config-if)#
R1(config-if)#ip add 192.168.110.1 255.255.255.0
R1(config-if)#exit

Configure Loopback ip on R2:

R2(config)#int lo0
R2(config-if)#
R2(config-if)#ip add 192.168.210.1 255.255.255.0
R2(config-if)#exit

Add loopback ip of R1 and R2 to OSPF network

R1(config)#router ospf 1
R1(config-router)#network 192.168.110.0 0.0.0.255 area 0
R1(config-router)#exit

R2(config)#router ospf 1
R2(config-router)#network 192.168.210.0 0.0.0.255 area 0
R2(config-router)#exit

You can reset and do a re-adjacency ospf with clear ip ospf process command on each routers :

R1#clear ip ospf process 
Reset ALL OSPF processes? [no]: yes
R1#

R2#clear ip ospf process 
Reset ALL OSPF processes? [no]: yes
R2#

But for this tutorial, reload both routers to see the changes of ospf process. Don’t forget to save the configuration first before reload.

R1#write
Building configuration...
[OK]
R1#reload
Proceed with reload? [confirm] y

R2#write
Building configuration...
[OK]
R2#reload
Proceed with reload? [confirm] y

After reload, check each router neighbor.

R1#show ip ospf nei

Neighbor ID      Pri   State      Dead Time   Address      Interface
192.168.210.1      1   FULL/BDR   00:00:36    12.12.12.2   GigabitEthernet0/0
R1#

R2#show ip ospf nei

Neighbor ID    Pri   State      Dead Time    Address      Interface
192.168.110.1    1   FULL/DR    00:00:33     12.12.12.1   GigabitEthernet0/0
R2#

As the output above, you can see the router-id on each routers are changes. Each routers are use the loopback ip address with highest value as Neighbor ID.

And Now let’s configure each routers with static router-id.

Configure Router-id on R1.  Once configured the router-id, router will tell you to reload the ip ospf process to take the effect. Then reload the process with clear ip ospf process command.

R1(config)#router ospf 1
R1(config-router)#router id
R1(config-router)#router 
R1(config-router)#router-id 1.1.1.1
% OSPF: Reload or use "clear ip ospf process" command, for this to take effect
R1(config-router)#exit
R1(config)#end 
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
R1#

Configure Router-id on R2.

R2(config)#router ospf 1
R2(config-router)#router-id 2.2.2.2
% OSPF: Reload or use "clear ip ospf process" command, for this to take effect
R2(config-router)#exit
R2(config)#end 
R2#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
R2#

Show the neighbor on each routers and see the router-id has changes.

R1#show ip ospf nei

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/BDR 00:00:31 12.12.12.2 GigabitEthernet0/0
R1#

R2#show ip ospf nei

Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:35 12.12.12.1 GigabitEthernet0/0
R2#