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RSTP

Hi I'm running RSTP on a 2950 switch

When executing the show spanning-tree command is tells me the Mzx age timer is 20 sec

My understanding was that the blocikng state didn't exist when running RSTP ????

per

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Re: RSTP

Hi Per,

RSTP is in fact build on the top of STP. It's true that timers have little use in a network properly configured for RSTP. However, there are some few cases where RSTP falls back into an STP like mode of operation and where the timers will be useful:

- when dealing with shared segment (it has been improved in the latest spec but not yet updated in our implementation).

- when interacting with legacy STP bridges.

About the max-age timer in particular.It's true that max-age will not be used directly to age out information (3xhello-time is used instead). However, the age of BPDU received on the root port are still increased by one when transmitted on designated port. As a result, in a pure RSTP network, the age is used as a kind of hop count, and max-age represent the maximum number of bridge a BPDU can go through before being discarded.

I did not really get your "blocking state" statement though. Let me know if you still have any doubt.

Regards,

Francois

1 REPLY

Re: RSTP

Hi Per,

RSTP is in fact build on the top of STP. It's true that timers have little use in a network properly configured for RSTP. However, there are some few cases where RSTP falls back into an STP like mode of operation and where the timers will be useful:

- when dealing with shared segment (it has been improved in the latest spec but not yet updated in our implementation).

- when interacting with legacy STP bridges.

About the max-age timer in particular.It's true that max-age will not be used directly to age out information (3xhello-time is used instead). However, the age of BPDU received on the root port are still increased by one when transmitted on designated port. As a result, in a pure RSTP network, the age is used as a kind of hop count, and max-age represent the maximum number of bridge a BPDU can go through before being discarded.

I did not really get your "blocking state" statement though. Let me know if you still have any doubt.

Regards,

Francois

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