How can I make a PVST+ switch root towards MST network?

Published on April 12 2014 by dellpe

Question:

In a network where the majority of the Cisco WS-C3750X-48P-S switches are running MST but a select few are still running PVST+, how can the PVST+ switch be configured to be the root of the spanning tree? Topology below:

+--------+             +---------+
|        +-------------+         |
|  SW1   |             |  SW2    |
+---+----+             +--+--+---+
    |                     |  |
    |       MST           |  |
    |                     |  |
    |                     |  |
    |     +----------+    |  |
    +-----+          +----+  |
          |   SW3    |       |
          +--+-------+       |
             |               |
             |               |
             |      PVST     |
             |               |
             |      +--------+-+
             +------+          |
                    |   SW4    |
                    +----------+

The PVST+ switch has the following configuration:

spanning-tree vlan 1-4094 priority 0

However the MST switch logs a message that PVSTSIM is failing:

%SPANTREE-2-PVSTSIM_FAIL: Blocking root port

Why is it behaving like this and what needs to be done to make the PVST+ side root?

Answer:

The way it works when connecting MST and PVST domain is that the MST switch at the edge of the domain will simulate PVST+ by sending same BPDU for all VLANs configured on the trunk towards the PVST+ switch.

The CIST replicates BPDU for every VLAN. Because there is not one instance per VLAN for MST the sys ID is set to 0 on these BPDUs. When connecting MST and PVST+ domain VLAN 1 is used to connect the domains.

When the PVST+ side sends a BPDU with priority 0 for VLAN 1 it is actually 1 because of extended system ID. If VLAN 2 is also configured then PVST+ side sends BPDU with ID of 2 because of the extended system ID.

The way PVST simulation works on MST switch is that the port can only have one role because there is not one instance running per VLAN. The CIST is now sending BPDU that it knows a root which has a priority of 1. This means that MST switch should be root forwarding but then it sees a BPDU with a priority of 2. Because the MST switch has better root information it should become designated forwarding on the port but the port can only have one role so the PVST simulation fails.

To make this work the PVST+ side must have worse priority on VLAN 1 than all the other VLANs on the trunk. The following configuration will work:

spanning-tree vlan 1 priority 4096
spanning-tree vlan 2-4094 priority 0

This way the PVST simulation will work and the following log message should be seen:

%SPANTREE-2-PVSTSIM_OK: PVST Simulation inconsistency cleared