Spanning Tree Protocol: MST and Non-MST interactions

In order for MST enabled devices to communicate with IEEE STP/RSTP a single MST instance is chosen for a specific port and represents all other MST regions configured on that port.  Messaging to PVST instances requires that the same port role choice is made across all VLAN STP instances on PVST devices, making the same consistent decision on what the port role will negotiate to across all PVST instances.

PVST Simulation allows for consistent operation between MST and PVST+ regions.

MST > PVST

In this direction MST chooses the IST as the representative, using IST information in all BPDUs generated out to PVST.  All PVST+ instances must receive the same IST information formatted in PVST+ BPDUs.  MST Boundary ports replicate the PVST+ BPDUs to all VLANs, this allows consistent information to be sent on all VLANs.  A PVST neighbor receiving those BPDUs on any port will make an identical consistent choice of that ports role and state for all VLANs.

PVST > MST

In this direction MST uses VLAN 1 as the representative of the entire PVST region.  MST must ensure that all VLAN 1s interaction with MST is consistent across all other VLANs.

This is where consistency citeria are listed for MSTs interaction with PVST:

  • PVST+ BPDUs for all VLANs arriving at a Designated boundary port must be inferior to its own BPDUs derived from IST
  • PVST+ BPDUs for VLANs other than VLAN 1 arriving at a root boundary port must be identical or superior to PVST+ BPDUs for VLAN 1

If either of these cases if the criteria for a particular port role is not met, the PVST Simulation process will declare a PVST Simulation inconsistency and will keep the port in a blocked state until the consistency criteria for the port’s role is met.

For superior BPDUs received on VLAN 1 MST will become non-designated and be put into a blocking state, there is no need to check for consistency in this instance since if all PVST VLANs declared this port to be a non-designated port, if for some reason the MST device were to declare it something different, then an PVST Simulation inconsistency event would be triggered and the port would be blocked regardless.

It is recommended when working in blended MST/PVST networks to ensure the MST switch’s IST VLAN attains root priority for all VLANs.

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