Configuring OSPF Timers in Cisco

Open Shortest Path First (OSPF) is a link-state routing protocol used to find the best path between a source and destination router using its own shortest path first). OSPF was developed by the Internet Engineering Task Force (IETF) as one of the Interior Gateway Protocols (IGPs). H. A protocol designed to move packets within a large autonomous system or routing domain. A network layer protocol that operates on protocol number 89 and uses AD value 110. OSPF uses multicast address 224.0.0.5 for normal communication and 224.0.0.6 for Designated Router (DR)/Backup Designated Router (BDR) updates.

OSPF Timers:

OSPF Timers are used to maintain relationships with a neighbor. There are two timers in OSPF. First is the Hello timer, which is the interval in which the hello packet is sent to the neighbor to maintain the relationship. Next, there is the dead timer, it is the interval in the router that breaks the relationship when hello packets are not received for the interval. OSPF timers depend on which interface is being used.

You can also change the value of these timers with the help of the command:

ip ospf hello-interval
ip ospf dead-interval

Consider this topology with neighborship between the routers already formed:

Now let's change the hello and dead interval on R1's f0/0 interface.

R1(config)#int f0/0 R1(config-if)#ip ospf he 
R1(config-if)#ip ospf hello-interval 20 
R1(config-if)#ip ospf dead-interval 20 
R1(config-if)#ip ospf dead-interval 80 
R1(config-if)#exit

But there's a catch, to form a neighborship between 2 routers they must have the same hello and dead intervals, so the neighborship breaks.

% OSPF-5-ADJCHG: Process 100, Nbr 192.168.1.2 
on FastEthernet0/0 from FULL to DOWN,

To create and the neighbor ship back. We have to execute the same command on R2 as we did on R1.

R2(config)# int f0/0 
R2(config-if)# ip ospf hello-interval 20 
R2(config-if)# ip ospf dead-interval 80 
R2(config-if)# exi R2(config)#

And the neighborship forms again.

% OSPF-5-ADJCHG: Process 100, Nbr 192.168.1.1 
on FastEthernet0/0 from LOADING to FULL,