STP: forward-delay-time

Answered Question
May 3rd, 2009

Hello

In STP for the first 20s (max-age) root bridge elections occur. In the same moment in which switch receives last BPDU which allow him to decide who is root bridge it also knows which are designated/non-designated ports. In this moment whole topology is converged. For all this swith have 20s. Why do we need additional 2*forward delay time(2*15s) to start forwarding packets ? Why do we need "listening phase" ? Why switch could not start forwarding/learning phase directly after topology converge ?

Thanx

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Correct Answer by Giuseppe Larosa about 7 years 7 months ago

Hello Maciej,

each phase needs to stay for at least the propagation time in a 7 hops network.

Because in STP 802.1D BPDUs are sent every hello interval 2 seconds it comes 15 seconds.

7*2 +1

The objective is to prevent bridging loop.

So the two phases:

listening

learning

and the port moves from one state to another after 15 seconds.

Two switches could take a wrong decision in a lan segment because not aware of another change/decision taken in another lan segment.

Hope to help

Giuseppe

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mlopacinski Sun, 05/03/2009 - 10:08

Thanx, but there is no answer for my question.

If topology is created durring blocking phase (or maybe i am wrong and it is not created durring blocking phase?) .

Why do we need to wait 30s more ? I now understeand that changing many port's status might create loops but why to wait 30 seconds more, why this value ? - why not 5s or 180s ?

Correct Answer
Giuseppe Larosa Sun, 05/03/2009 - 11:27

Hello Maciej,

each phase needs to stay for at least the propagation time in a 7 hops network.

Because in STP 802.1D BPDUs are sent every hello interval 2 seconds it comes 15 seconds.

7*2 +1

The objective is to prevent bridging loop.

So the two phases:

listening

learning

and the port moves from one state to another after 15 seconds.

Two switches could take a wrong decision in a lan segment because not aware of another change/decision taken in another lan segment.

Hope to help

Giuseppe

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