MST vs. PVST+ compatibility

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Feb 2nd, 2010

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We are going through a major change in our network in that we are replacing our current 6509's with the ASR 9010's. All of our access switches are multi-homed to two separate distribution switches and are heavily dependent on spanning-tree to prevent loops as well as provide redundancy in the event of a link failure. We currently run PVST+ on all of our access switches, however, the ASR 9000's only supports MST which can lead to issues with our legacy switches. We have quite a number of access switches which has the option to run various flavors of STP; which Ill break out into several parts in order to describe our dilemma.

Our 2900XL and 3500XL switches:

The Cisco XL series is only capable of PVST+ and not MST. I understand that MST is somewhat backwards compatible with PVST+, however, what would Cisco recommend we do for these switches for interoperability with MST? Do we leave them as is and depend on the backward compatibility mechanism? What must we do to achieve stability and resiliency between the two different STP?

Our 4003’s and 4006’s switches:

Our 4000’s series switches only support PVST+, MISTP, MISTP-PVST+. I would assume our best choice here is to run MISTP since it is the pre-cursor to MST. However, as I understand it, MISTP does not incorporate RSTP (802.1w) as MST does. Therefore, my question would be what are the ramifications of running MISTP with MSTw/RSTP? Again, what would Cisco recommend for us to do?

Our 6500’s:

In order to prep for the ASR’s, we will need to convert the STP on all our MST capable switches to MST. This means that the root for all our MST will be our 6500’s. My question would be is the MST implemented on the 6500’s going to be the same version as the MST on the ASR series?

Im looking for any input the forums can provide or past experiences in working with MST and PVST+ simultaneously.

I have this problem too.
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Ganesh Hariharan Tue, 02/02/2010 - 22:42

We are going through a major change in our network in that we are replacing our current 6509's with the ASR 9010's. All of our access switches are multi-homed to two separate distribution switches and are heavily dependent on spanning-tree to prevent loops as well as provide redundancy in the event of a link failure. We currently run PVST+ on all of our access switches, however, the ASR 9000's only supports MST which can lead to issues with our legacy switches. We have quite a number of access switches which has the option to run various flavors of STP; which Ill break out into several parts in order to describe our dilemma.

Our 2900XL and 3500XL switches:

The Cisco XL series is only capable of PVST+ and not MST. I understand that MST is somewhat backwards compatible with PVST+, however, what would Cisco recommend we do for these switches for interoperability with MST? Do we leave them as is and depend on the backward compatibility mechanism? What must we do to achieve stability and resiliency between the two different STP?

Our 4003’s and 4006’s switches:

Our 4000’s series switches only support PVST+, MISTP, MISTP-PVST+. I would assume our best choice here is to run MISTP since it is the pre-cursor to MST. However, as I understand it, MISTP does not incorporate RSTP (802.1w) as MST does. Therefore, my question would be what are the ramifications of running MISTP with MSTw/RSTP? Again, what would Cisco recommend for us to do?

Our 6500’s:

In order to prep for the ASR’s, we will need to convert the STP on all our MST capable switches to MST. This means that the root for all our MST will be our 6500’s. My question would be is the MST implemented on the 6500’s going to be the same version as the MST on the ASR series?

Im looking for any input the forums can provide or past experiences in working with MST and PVST+ simultaneously.

Hi,

As the per the post The compatibilty of MST running switches with PVST+ switches will work as MST is backward compatible with PVST+, Rapid-PVST+, and Prestandard MST (MISTP). MST switch is connected to the other STP (PVST+ and Rapid-PVST+) switches by the Common Spanning Tree (CST). Other STP (PVST+ and Rapid-PVST+) switches see the entire MST region as a single switch. When you connect the prestandard MST switch with the standard MST switch, you need to configure spanning-tree mst pre-standard in the interface of the standard MST switch.

Hope that help

If helpful do rate the post

Ganesh.H

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