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ovt Bronze

MST - PVST+ interoperability


It is well known that the MST boundary switch replicates the IST BPDU (instance 0 BPDU) on all VLANs to send them to a PVST+ neighbor. The MST switch does this because PVST+ switch _expects_ to receive a BPDU on every VLAN.

The question is: Does this mean that replicated BPDUs contain exactly the same information? What about extended system id in the RootID field? Does the MST boundary switch set extended sys-id in the PVST+ BPDU to 1, 0 or a VLAN number?

This is important thing to know in case the root is located outside of the MST region (i.e. the PVST+ switch is the root).

For example, suppose we have a triangle - 1 MST switch and 2 PVST+ switches.

One of the PVST+ switches has priority 0+1 for VLAN 1 and 0+10 for VLAN 10 (0 + ext. sys-id) and is the root for both VLAN 1 and 10.

This (first) root PVST+ switch sends VLAN 10 BPDU to the MST boundary switch with priority 0+10.

If the MST switch _always_ set priority to 0+1 in the PVST+ BPDUs replicated to the second PVST+ switch that switch will think that the root is accessible thru the MST region.

Then the second PVST+ switch will send the BPDU to the first PVST+ switch and this switch will trust it. It will think it is no longer the root for VLAN 10 (although it will see its own MAC in the root BID field!).

This will lead to a conflict: something like "the root for VLAN 1 is outside of the MST region and root for VLAN 10 is inside the MST region". The MST boundary port will be blocked as the "root inconsistent port".

New Member

Re: MST - PVST+ interoperability

Privet Oleg!

That?s true, MST region replicates the IST BPDU on all VLANs to simulate a PVST+ neighbor, in the enclosed link Cisco use the word (duplicate) but you know, logically each PVST+ will expect and deal only with BPDU with the good ext.system ID. So I think, each VLAN will receive the IST BPDU with its own VLAN ID.

- By the way in the case of PVST+ the bridge ID = Bridge Priority(4bits) + ext.system ID (12bits) + MAC(6bytes), and the Bridge priority cannot be 0 but minimum value is 1, this mean that the Bridge Priority+ ext.system can only be an increment of 4096.

Cisco provides strict recommendations to implement MST:

-1- if MST bridge is the root, it must be the root for all VLANs (RECOMMENDED).

-2- if PVST+ is the root it should be the root for all VLANs including CST which use VLAN 1 (NOT RECOMMENDED AND ALL OTHER TOPOLOGIES ARE CONSIDERED AS INVALID).

-3- The topology will fail and produce an error message if the MST bridge is the root for the CST while the PVST+ bridge is the root for one or more VLANs. This puts the boundary port in root-inconsistent mode, because MST boundary ports can forwarding or blocking for ALL VLANs (Probably your case).

Here is an excelent link about 802.1s:

Good work!


New Member

MST - PVST+ interoperability

Thanks Abdel for your assistance

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