Spanning-Tree MST “Shr Bound(PVST)” Tree

I have came across an issue lately where one of my customers was reporting that a specific port on their switch while running the command show spanning-tree vlan # shown a different port-type than the rest of them.

The output was something like:

SW1#sh spanning-tree vlan 13

MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address aabb.cc00.0600
This bridge is the root
Hello Time 2 sec Max Age 30 sec Forward Delay 15 sec

Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address aabb.cc00.0600
Hello Time 2 sec Max Age 30 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–

Et3/3 Desg FWD 2000000 128.100 Shr
Po12A Desg FWD 1000000 128.515 Shr Bound(PVST)

Customer was concerned about it and if it could create loops on the l2 network.

The flag being shown there basically let us know that the Port-Channel 12 has determined that it connects to a device running PVST so its basically an MST Region Boundary Port.

As simple as that.

MST is able to identify that a neighbor switch is running PVST as soon as it determine the neighbor it’s on a different region or if it receives legacy 802.1d BPDUs from that neighbor.

This does not means that a loop will happen as this 2 protocols can co-exist but if you have access to the other box you can go ahead and enable MST on the other side.

As soon as you do that you should see the following output instead:

SW1#sh spanning-tree vlan 13

MST1
Spanning tree enabled protocol mstp
Root ID Priority 24577
Address aabb.cc00.0600
This bridge is the root
Hello Time 2 sec Max Age 30 sec Forward Delay 15 sec

Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address aabb.cc00.0600
Hello Time 2 sec Max Age 30 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–

Po12A Desg FWD 1000000 128.515 Shr

Regards,

iNetworks