Convergence based on hello and dead timer
-Suports sub-second timers
Different timers for different network types
-#Sh ip ospf Interface
Changing hello time automatically adjusts dead time
-# ip ospf hello-interval
-# ip ospf dead-interval
The OSPF timers are mainly going to be based on how long before we can detect the neighbor is down, then how long until we can flood the LSU and then run the SPF.
One problem we can run into is if the layer two link status to the neighbor is not a good indication of the connectivity - we would then need to reply on the layer 3 timers to figure out if they're down.
The other issue we run into if we have a switch in the middle is we don't have a direct link to the line protocol status of the other link side because of the layer two path.
Ideally - before implementing OSPF, we would want to implement some sort of mechanism that can keep track of the layer 2 status end to end of the neighbors. Typically BFD - bidirectional forward direction is used for this.
This will enable an additional keep alive on the Ethernet link between tow neighbors.
Ospf Timers - by default
FastEthernet0/0 is up, line protocol is up
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 -- By default it will take up 40 seconds to find out if the neighbor is down.
Note: Also if we lower the timers it's very process or intensive for the control plane.
If we wanted to change the hello -interval we can do the under the interface.
We can under the interface -
R5(config-if)#ip ospf hello-interval ?
to say one second.
We can also change the dead-interval to minimal - which will by default change the hell-timers to one second.
R5(config-if)#ip ospf dead-interval minimal
R5(config-if)#ip ospf dead-interval minimal hello-multiplier ?
<3-20> Number of Hellos sent within 1 second
The dead interval minimal will change the hello timer to one second, the hello-multiplier - is how many hellos in time will we send within the second.
Sh ip OSPF interface will show what the timer will be on the link. This is also negotiated during the adjacency so bot sides do need to agree to form the ADJ.