rstp with pvst+

Answered Question
May 17th, 2009
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Hi. I just wish to ask what will happen if one switch running pvst+ is connected to a switch running rstp? Or pvst+ and mst? Or rstp and mst? Redundant connections between the two switches and each share the same vlans.


Thanks in advance.

Correct Answer by Jon Marshall about 8 years 1 week ago

If SW1 and SW2 have exactly the same vlans then all vlans would revert back to pvst+ and use standard 802.1d timers.


STP process would not fail through any combination because RSTP is backwards compatible with PVST.


As an example in the real world. Lets say you have a switched network consisting of 20 switches only 1 of which is running pvst+. Obviously you would upgrade the switch but lets say it's not possible at the moment.

There are 50 vlans in use.


So if you allow all 50 vlans across all switches then all vlans will have to use 802.1d timers.

However let say all you need is 3 of the 50 vlans on the switch running pvst+. If you only allow those 3 vlans on the trunk between this switch and the one it connects to then those 3 vlans will use 802.1d timers. But the other 47 vlans will get the full benefit of rstp timers.


Jon

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Jon Marshall Sun, 05/17/2009 - 12:51
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Mark


I'm not that familiar with mst but i can help with the pvst+ to rstp.


SW1 runs rstp

SW2 runs pvst+


SW1 & SW2 are interconnected with a L2 trunk.


SW1 has vlans 2, 3, 4

SW2 has vlans 3, 4, 5


For vlan 2 rstp is in use withe rapid convergence


For vlans 3 & 4 pvst+ timers are in use as is the case for vlan 5.


Basically if a vlan exists on interconnected switches that are only running rstp you get the full benefit of rstp.


If any of the interconnected switches are running pvst+ then the vlan will fall back to 802.1d timers.


As MST uses rstp I suspect the above also applies.


Jon



marcusbrutus Sun, 05/17/2009 - 13:01
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Hi Jon,


Thanks for the reply.


What if sw1 and sw2 both have the same vlans? Will an rstp switch automatically revert back to pvst+ or will the stp process fail?

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Jon Marshall Sun, 05/17/2009 - 13:08
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If SW1 and SW2 have exactly the same vlans then all vlans would revert back to pvst+ and use standard 802.1d timers.


STP process would not fail through any combination because RSTP is backwards compatible with PVST.


As an example in the real world. Lets say you have a switched network consisting of 20 switches only 1 of which is running pvst+. Obviously you would upgrade the switch but lets say it's not possible at the moment.

There are 50 vlans in use.


So if you allow all 50 vlans across all switches then all vlans will have to use 802.1d timers.

However let say all you need is 3 of the 50 vlans on the switch running pvst+. If you only allow those 3 vlans on the trunk between this switch and the one it connects to then those 3 vlans will use 802.1d timers. But the other 47 vlans will get the full benefit of rstp timers.


Jon

carl_townshend Tue, 05/26/2009 - 01:11
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hi there


does this have to happen all the way to the root ? so if there are 3 switchs daisychained, it will put the mode to pvst for the particular vlans on each link all the way to the root switch?


cheers

glen.grant Tue, 05/26/2009 - 03:20
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Yes all the way back to the root , it is per vlan spanning tree . You can't have different timers for a given vlan even if its on a different switch . I would make up a plan on when you can convert all 3 at once to rpvst .

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