stp blocked port

Answered Question
Sep 13th, 2008

hi every body!

let say sw1 f0/2 is connected to sw2 f0/2 which is connected to root switch.sw1 has f0/3 as blocked port which is connected to sw3 f0/3 ,connected to root switch.

if sw2 loses fo/2, will the blocked port f0/3 transition into listening state after receiving bpdu from designated port or will it wait for bpdu with tc=1 and then transition into listening state?

simply put,would a blocked port transition after receiving bpdu from desiagnated port or after receiving bpdu with tc=1 from desinated port?

thanks a lot and have a nice weekend!

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Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Sarah,

in standard classic STP the switch will :

start to move the second port to listening state then to learning state and then to forwarding state if the state of the current root port is down.

If the current root port is still up but it has stopped to receive BPDUs another safety mechanism is used:

the max-age timer 20 seconds is waited. If after max-age the port isn't still receiving BPDUs, the port starts to send its own BPDUs.

So in this second case convergence time can be up to 50 seconds, in the first case some more then 30 seconds.

classic STP does not wait only for three intervals without receiving BPDU (like a RIP protocol does for example or like RSTP does more important)

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sat, 09/13/2008 - 09:15

Hello Sarah,

in standard classic STP the switch will :

start to move the second port to listening state then to learning state and then to forwarding state if the state of the current root port is down.

If the current root port is still up but it has stopped to receive BPDUs another safety mechanism is used:

the max-age timer 20 seconds is waited. If after max-age the port isn't still receiving BPDUs, the port starts to send its own BPDUs.

So in this second case convergence time can be up to 50 seconds, in the first case some more then 30 seconds.

classic STP does not wait only for three intervals without receiving BPDU (like a RIP protocol does for example or like RSTP does more important)

Hope to help

Giuseppe

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