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Beginner

Spanning-Tree : MST & PVST

I am looking to connect my Layer 3 switches running PVST to my customer's L3 switches running MST. The customer is configuring themselves to be the root.

Has anyone seen any issues with interoperability with PVST -> MST?

Thank you in advance!

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Hall of Fame Cisco Employee

Hello Ashley,

Yes, there can be issues caused by the so-called PVST Simulation run by Cisco switches that tries to emulate the whole MST region as a single PVST+ switch identically configured for all VLANs. The details are intricate and unless you are strongly interested in knowing the nitty-gritty details, let me just suggest a workable setup without going too deep.

The most sensible and straightforward way of making the MST and PVST+ interoperate is to make sure that inside the MST region, the root switch for the IST = MSTI 0 instance has the lowest numerical priority (e.g., 0) among all switches and VLANs whatsoever. In other words, you must make sure that the priority setting of the IST root switch is numerically lower than the priority of any switch in the PVST region in any VLAN. Because the PVST Simulation takes the data from the IST root and replicates it into PVST BPDUs on boundary ports, this setting will make the MST region win as a root switch for all VLANs in the PVST region, and will allow the MST and PVST to operate together nicely.

Best regards,

Peter

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Hall of Fame Cisco Employee

Hello Ashley,

Yes, there can be issues caused by the so-called PVST Simulation run by Cisco switches that tries to emulate the whole MST region as a single PVST+ switch identically configured for all VLANs. The details are intricate and unless you are strongly interested in knowing the nitty-gritty details, let me just suggest a workable setup without going too deep.

The most sensible and straightforward way of making the MST and PVST+ interoperate is to make sure that inside the MST region, the root switch for the IST = MSTI 0 instance has the lowest numerical priority (e.g., 0) among all switches and VLANs whatsoever. In other words, you must make sure that the priority setting of the IST root switch is numerically lower than the priority of any switch in the PVST region in any VLAN. Because the PVST Simulation takes the data from the IST root and replicates it into PVST BPDUs on boundary ports, this setting will make the MST region win as a root switch for all VLANs in the PVST region, and will allow the MST and PVST to operate together nicely.

Best regards,

Peter

View solution in original post

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Thank you, Peter.

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