Bridging Spanning tree domains (MSTP/PVST/RSTP)

[jack.riley1]

We have a scenario where we have VLANs spread through VDCs on a pair of 7Ks, one VDC running MSTP with 3 instances and another running RSTP. We are then connecting some 3rd party switches for a DC environment that are running PVST. 

Root guard triggered when plugging these switches in, even though the bridge priority for the root is 2 on the root bridge for some of these VLANs.

My main questions are;

How will the root election work for these switches across multiple instances of spanning-tree?

Does the elected root bridge election propagate across all spanning tree types (MSTP/PVST/RSTP) regardless? (except MSTP which separates these out by design)

Is there a way of preventing a bridge from attempting to become a root bridge on a particular interface, without affecting it's ability to trunk? 

KR - Jack

[gs.skills]

Hello,

For question 1,2 and about the rootguard triggered, even if you set the priority to the lowest possible value, there could still have a 3rd party switch set with the same value and a lower MAC address would make a better BID, so it requires to verify bridge priority on 3rd switches.(The lowest BID will be the root for the CIST and PVST): spanning-tree mst 0 priority 0

From the following link, i understand that the root bridge must be in the MSTP domain for all vlans and that root bridge in PVST domain is not supported:

http://blog.internetworkexpert.com/2010/02/22/understanding-mstp/

For the question 3: when a switch boot, from his point of view he is the root until another switch says otherwise. Maybe you could let him believe he is the root with bpdufilter... but it breaks the purpose of STP and requires to remove any physical loop.

I hope it will help you,

Regards, Guillaume