spanning tree modes

Unanswered Question
Apr 22nd, 2009

Hi all, If I have a distribution switch that is running RSTP, But I have uplinks of this that run RSTP and STP, Will the dist root switch send out STP bpdus for the switches that run normal STP, Is this done automatically, and will it show that it is running STP and RSTP ?

please help

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Giuseppe Larosa Wed, 04/22/2009 - 03:42

Hello Carl,

fallback to legacy STP is done on per link basis.

show spanning-tree interface type x/y should give the required information

Hope to help


carl_townshend Wed, 04/22/2009 - 07:55

so, what will it use as the root switch, will it take the root switch that is currently set up for RSTP ? and will the root bridge send STP bpdu's ?

Giuseppe Larosa Wed, 04/22/2009 - 09:18

Hello Carl,

on a per link ? per port basis fallback to STP is performed.

if Rapid STP best candidate is better then any legacy STP bridge-ids the root bridge will be in the rapid STP portion of the network.

you may want to use spanning-tree priority commands to ensure who are the root bridge and the backup /secondary root bridge.

Election happens for each instance/vlan.

Let's suppose the root bridge is in the rapid STP portion of the network it will send out legacy STP BPDUs only out those links/ports leading to switches running PVST.

The same will happen on other switches in the rapid STP portion of the network.

this allows for smooth migration.

Hope to help


carl_townshend Thu, 04/23/2009 - 02:23

hi there,

So does the RSTP bridge see a normal STP bpdu and automatically send out a normal STP Bpdu along that link?

when I do I show spanning-tree, will it show as only running in rstp mode, but to see the stp ports, I have to do a show stp interface detail ?


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