forward delay time in STP

Unanswered Question
Apr 1st, 2009

Hi all

I read that the forward delay time for listening and learning is 15 secs, is this 15 seconds each, so 30 in total, which seems more real ?

I have this problem too.
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Giuseppe Larosa Wed, 04/01/2009 - 05:26

Hello Carl,

it is 15 seconds timer used two times.

by the way for the other thread:

this 15 seconds accomodates for

7 switch hops with an hello interval of two seconds:

15 = 7*2 + 1

So in legacy 802.1D if the diameter is increased over 7 hops the timer has to be changed accordingly.

the forward timer must be greater then the time it takes a message to travel on the whole tree

Hope to help


carl_townshend Wed, 04/01/2009 - 07:14

so are we talking about the listening and learning timers? if so, when you change the forward delay to 20 secs, would this be 20 for each ?

also when the bpdu is sent from the root, the other switches forward this on, so it will get forwarded every 2 secs passed on from the switch before?

also am i right in saying it is different in RSTP, as every switch generates its own BPDU ?



Giuseppe Larosa Wed, 04/01/2009 - 07:54

Hello Carl,

your understanding is correct or at least I agree with you

Hope to help



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