Conversion from PVST+ to MST

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May 4th, 2007

We are looking at converting our 6509 to MST. I would like the whole network to be converted to MST but we have alot of 3548xl that I understand do not support MST. Is it possible to have a mixed environment? Are there any possible issues in having a mixed environment or should we first look at replacing the 3548xl's first ad converting the whole nextwork to MST?

I have this problem too.
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ankbhasi Fri, 05/04/2007 - 05:53

Hi Allen,

Yes it is possible and MST is backward compatible with PVST+. PVST+ switches see the entire MST region as a single switch.

Have a look at this link which will guide you with configuration example to migrate from PVST+ to MST

Also read this link for MST interaction to outside world



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p-allen Fri, 05/04/2007 - 07:56

thanks for the reply. I also asked our consultant and here are their responses.

I?d recommend against the mix mode environment, especially with the older hardware. Personally, I?d phase out the older hardware first and then migrate over to rapid pvst. In addition, I bounced this off several other engineers and here is their feedback:


?MST does recognize boundary ports automatically. MST propagates the "common spanning-tree" (CST) that spans all MST and non-MST areas.

But, ask the client why they want to use MST. It's only required in environments with many hundreds of VLANs, and its configuration is far more complex with no room for inconsistency or errors.

Also, be aware that older software has a pre-standard version of MST that should be avoided.?


?It is possible to have a mixed environment, assuming there aren?t any PVST switches in between MST switches. Additionally, you?ll want to have the MST as the root bridge of the spanning tree domain. MST gets weird when PVST is the root bridge. ?


?I would STRONGLY suggest you do your best to persuade the customer that this is a poor idea. The only time MST is really beneficial is when you have 25-30+ VLANs on the same layer 2 domain. STP chatter just isn?t significant enough to warrant the extreme complexity of an MST configuration. Additionally, you would never want to do it unless your entire environment could support it. Also, in cases where there were that many VLANs that would warrant MST, a layer 3 to the closet redesign would be MUCH more in order.

MST uses rapid spanning tree (RSTP) as the underlying technology. Not only do the 3548xl switches not support MST, they do not support RSTP. Their network would be relegated to the least common denominator which is regular old STP (802.3d) so this would be a huge complex undertaking that would not really get them anything except a network that would be horrible to troubleshoot. In my opinion they would most likely break stuff before fixing it.?


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