MSTP backward compatibility ?

I have a question regarding MSTP backward compatibility with PVST. According
to Cisco documentation "MSTP maintain backward compatibility with equipment
that is based on PVST+"

I am testing this in the lab and and it does not work. In the topology below
I have MSTP and PVST switches connected. I tried several combination of stp
priority on different switches and it looks like MST and PVST switches will
not exchange stp priority at all.

Here is the scenario:

SW1=======SW2 ---> MSTP
|| \\ // ||
|| \\// ||
|| //\\ ||
|| // \\ ||
SW3=======SW4 ---> PVST

For example, if I configure SW4 to be root bridge for vlan 10 I will end up
with two switches claiming to be root for vlan 10: SW1 and sw4 will claim to
be root.

It looks to me that MST and PVST switches do not exchange stp priority, any
ideas about "backward compatibility" ?

BTW, I tested a mix of RSTP and PVST+ in the similar topology and it worked
OK.

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Received on Tue Mar 01 2011 - 15:33:32 ART