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How does MISTP-PVST interoperate with PVST+



I've the problem that my CAT6K-Switches with Sup1A are facing a very high CPU-Load caused by BPDU-Traffic of about 400 VLANs. I wanted to migrate to MST but because I will still have PVST+ Switches, and it cannot be garantied that there is only one Root-Bridge for all PVST+-VLANS, this is not my solution.

Now since my Cat6k are running on CatOS 8.4(3) I also have the possibility to run MISTP-PVST-mode. Unfortunately I cannot see anywhere how it exactly works to decide whether this can solve my problem or not.

The network has to be very flexible, thus I have to have these many VLANs!

Thanks in advance


2 Replies 2


MISTP (802.1s) is an IEEE standard which allows several VLANs to be mapped to a reduced number of spanning-tree instances. This is possible since most networks do not need more than a few logical topologies. Each instance handles multiple VLANs that have the same Layer 2 topology.

Per VLAN Spanning Tree Plus (PVST+) provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification and is not supported on non-Cisco devices.

To view about the spanning tree instance, use the following command on any of your cat6k switches.The o/p will be as below.From this o/p, i think u may be able to identify its overall functionality.

show spantree mistp-instance active

Instance 1

Spanning tree mode MISTP

Spanning tree type ieee

Spanning tree instance enabled

Designated Root xxxxxx

Designated Root Priority 32769

Designated Root Cost 0

Designated Root Port none

VLANs mapped: 1

Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec

Bridge ID MAC ADDR xxxxxxx

Bridge ID Priority 32769

VLANs mapped: 1

Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec

Francois Tallet
Rising star
Rising star

Please please, don't run MISTP;-)

There is a common confusion between MST and MISTP.

MISTP is a cisco proprietary ancestor of MST. MST is an IEEE standard that I strongly recommend you consider (MISTP will be dropped as soon as we can).

I put a white paper on CCO giving an overview of MST (you might want to read the RSTP white paper also in order to follow:



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