The MSTP bridge simply simulte all PVST+ BPDUs by replicating CIST BPDUs on the Boundary link between the PVST+ bridge and the MSTP domain on the Trunk link between them. The MSTP bridge recieves all PVST+ BPDUs and process them using Internal Spanning-tree the (IST). The MSTP bridge view like another PVST+ domain to the PVST+.
The Cisco's Implementation recommends to have always the root bridge for all VLANs to be the CIST root bridge inside the region.
Regards,
Mohamed
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