Spanning-tree Question

Unanswered Question
Jun 4th, 2008

What are the negative consequences associated with running pvst and rapid-pvst in the same environment?

I have this problem too.
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Jon Marshall Wed, 06/04/2008 - 09:55


No major ones as such but you need to be aware that if any vlan that exists on a switch running PVST and another switch running Rapid-PVST will use standard 802.1d timers.


jrabener Wed, 06/04/2008 - 09:59

Thanks for your reply Jon.

So, if your looking for the benefits associated with rpvst, they will not be implented in a "mixed" environment.


Jon Marshall Wed, 06/04/2008 - 10:04

Kind of. It depends on whether you need your vlans on all swithes. So lets say you have

SW1 -> SW2 -> SW3

Each switch is connected via a L2 trunk. SW1/SW2 run rapid-pvst. SW3 runs PVST.

If you have a vlan eg vlan 10 that needs to be on all switches then yes the faster convergence of rapid-pvst will not be realised.

However if you know that vlan is not needed on SW3 and you clear vlan 10 off the trunk between SW2 & SW3 then you would get the benefits of rapidp-pvst for vlan 10.

So it depends on how you organise your vlans and their distribution.


Francois Tallet Wed, 06/04/2008 - 12:52

To be a little bit more precise about the consequence of RSTP/STP operation. Ports at the boundary between the RSTP/STP region might block as a result of an RSTP convergence. They will take 30 seconds to unblock, while the RSTP region will reconverge instantly. So it's not obvious to evaluate the benefit/cost of this situation. The RSTP part of the network will converge must faster if it experiences a failure, but the reconverge will disconnect the STP part for 30 seconds. I guess that if you have few STP bridges at the edge, it might still be worthwhile.



Jon Marshall Wed, 06/04/2008 - 12:55


Is it still valid to say that

a) If you restrict a vlan to switches running only RSTP and clear them off trunks attached to STP switches then convergence for these vlans will use RSTP timers ie. almost instantaneous.

b) If you have a vlan that spans both RSTP and STP switches it can only converge to STP timers.


Francois Tallet Wed, 06/04/2008 - 13:05

Hi Jon,

Yes your point is valid because in case a), there is no boundary RSTP/STP (so no interaction between the two versions).

I just wanted to precise the consequence of b) because it's difficult to evaluate what's best.

In redundant STP network, a failure can be repaired in 30 seconds. During these 30 seconds, the network is partitioned.

In an RSTP/STP network, a failure in an STP part will converge just like STP. However, a failure in the RSTP part will be repaired instantly within the RSTP region (in that aspect it's better), but the convergence will most likely introduce a 30 seconds cut between the RSTP and the STP regions.

This should help deciding whether it is worth migrating to a partial RSTP network or staying with a full STP network. The choice is not obvious.

If all the devices can be migrated to RSTP, then the choice is obvious;-)



Jon Marshall Wed, 06/04/2008 - 13:11


Many thanks for that, just wanted to make sure i wasn't misleading anybody :-)



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