We got one cloud of metro Ethernet consist of 2 unit of 7613 as NPE. Both have serve 5 customers and each of customers are connected via UPEs in separate ring to the both NPE. The issue is both NPE are running MSTP.

For instance 0 = all vlans

instance 1 = vlan 100

instance 2 = vlan 200

All UPEs except 1 ring of customers are using PVST.

That 1 ring customer are using MSTP same like both NPEs.

It happens that trunk between the NPE_A & NPE_B are disconnected.

All customers UPE that running PVST are experience port status "BLK" at the each ring .

Except the one UPE that running MSTP, all UPE in the ring are forwarding.

Why this issue happen in customer that tunning PVST?

Any interoperability issue between PVST & MSTP?