05-03-2009 06:38 AM - edited 03-06-2019 05:30 AM
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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05-03-2009 11:27 AM
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
05-03-2009 07:27 AM
05-03-2009 10:08 AM
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 ?
05-03-2009 11:27 AM
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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