OSPF fast hello is used for fast OSPF conversion. it is supported from OSPF version 3 (IOS XE).
OSPF fast hello packets means , hello packets will be send at the interval of less than one second. We can configure it by using below command:
Router(config-if)#ip ospf dead-interval minimal hello-multiplier 5
Above command will be configure on interface as you can see , most of OSPF version 3 commands need to be configure on interface.
Hello-multiplier value is used to define that how many hello packets can be received in the time which is specified for dead interval.
Default dead interval is 1 second. so if we are using above command that means we are assuming 5 hello packets to be receive in 1 second , and for OSPF neighbor-ship continuity minimum 1 hello packet should be receive. otherwise OSPF will terminate the neighbor-ship.
So hello-multiplier is used to specify the number of hello packets which can be receive under dead interval. Default dead interval is 1 second but it can also be change if required.
We have configured the OSPF fast hello packets on both Router A and Router B as below
RouterA(config-if)#ip ospf dead-interval minimal hello-multiplier 1
RouterB(config-if)#ip ospf dead-interval minimal hello-multiplier 1
As per above configuration , at least one hello should be receive in a second. If Hello packet not received in 1 second on Router A from Router B. Then Router A will terminate the OSPF neighbor-ship with Router B
This feature is useful when routers are connected in LAN and locally. Suppose we are having multiple routers and switches in our core network and all the devices having OSPF neighbor ship over LAN. We are also sure , here will not be a network interruption as well as hardware related interruption like high CPU or high memory utilization. in this type of network we should configure the fast hello feature. so we can easily and quickly detect a neighbor-ship failure.
If you are having high CPU utilization on devices or high memory utilization or low bandwidth links or high bandwidth utilization in your network then you should avoid this feature.
This is only good for core networks as i explained above.
You can also experience fake alerts or OSPF neighbor-ship flap in also a well designed core network . Because one hello packets can be drop due to any reason. So you should increase the hello-multiplier value as per your requirement. I have also experience the same in real environment.
Thanks For Reading………………….
I will also suggest you to read below post: