Spanning Tree MST mode

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I currently have a single Cisco 4506 that is the STP root for Vlan 65. I have another Cisco 4948 that is the STP root for Vlan 64 and they are obviously trunked (802.1Q). Both switches are in PVST mode.

If I switch the mode to MST will I temporarily lose connectivity between the switches? Thanks in advance for any help you might provide!

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smalkeric Thu, 05/24/2007 - 05:20
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When the switch is operating in PVST mode, loop guard prevents alternate and root ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.

Francois Tallet Thu, 05/24/2007 - 10:47
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Yes, you will get temporary loss of connectivity. When we change the spanning tree mode, we start by putting all the ports into discarding state, then STP is stopped and restarted in the new mode. This new mode starts from scratch and moves the ports back to forwarding. If you only have two switches connected the one to the other via a trunk, there is nothing much you need to configure for STP. If you move both switches to mst mode at the same time, with no configuration, you can expect few seconds of traffic disruption.

That should be relatively plug and play (however, I would recommend you check the status of the trunk on each bridge, to make sure that they don't end up in PVST simulation mode. In which case I would recommend a shut/no shut or simply a "clear spanning-tree detected-protocol).



pocketldq Thu, 05/24/2007 - 19:51
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Hi all !

have U ever used MST with many products. I mean that I have 2 Cisco 6509E, 8 cisco 4507R, many switch cisco 3030 ( module switch ) Nortel module switch and DELL switch. Nortel switch support PVST but DELL not. I'm considering the convergence time. I intend to create 2 MST region and C6509 is root each region.

I need some advies.


Francois Tallet Fri, 05/25/2007 - 07:53
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PVST and rapid-PVST are cisco proprietary. If you want to interoperate with a third-party MST or RSTP implementation, use MST on the Cisco switches (MST interacts gracefully with RSTP, that's why we did not implement RSTP specifically).

Can you clarify why you want to use two different regions? It's probably not necessary.




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