OSPF fast hello query

[tinu_karki]

Had a question regarding OSPF fast hello. Lets say i configure fast hello on an interface using the command:

ip ospf dead-interval minimal hello-multiplier 5

This means my dead interval is 1 second (this value never changes)

This also means 5 hellos are sent out within 1 second which equals a hello message every 200ms

I have two question

1. Firstly dead interval doesnt follow the normal behaviour of 3 times the hello interval, with fast hellos dead interval is set to 1 second irrespective of hello interval - is this correct?

2. If dead interval is set to 1 second the what is the point of specifying the hello multiplier ie, does it really matter what the multiplier is set to? Even if it set to 100ms per hello or 200ms per hello dead interval will not happen until 1 sec.

[Henrik Grankvist]

Hi

1. Correct

2. Yeah... but it makes it possible for the router to "miss" some hellos without tearing the connection down. Because it is alot of hellos so the chanse is greater that it will miss some hellos over a congested link.

Anyway, it's a cool feature

[tinu_karki]

Thanks Henrik,

Still confused on point 2 do you mean, the more hello's the worse as hellos might be missed? What is the best practise in terms of the number of hellos to configure for 1 second?

[vijay.swaminathan]

by default, Cisco recommends  to have the Dead times 4 times the Hello timers.

In this case, if the dead timer is 1 sec, then the hello packets are sent every 250 ms.

The reason why we set the dead timers to be 4 times the hello timers is , even if we miss out one hello packet, the adjacency/neighborship should not go down. so we will wait for 4 hello's to elapse before we declare the neighborship as down.

HTH

-Vijay

[tinu_karki]

Hi Vijay,

Normally the dead timer is 3 * the hello timer however my assumption was that with fast hellos the dead timer never changes and is always 1 second?

[Henrik Grankvist]

Yes, always one seconds.

Hello timer is then technically 0 seconds when you user fast hellos. So even if you confiure 4 hellos/sec on one router the other router can have 5 hellos/sec and adjecency will still form (equal hello/dead timer is a mandatory requirement for adjecency to form in OSPF (except for this)).