Re: MSTP backward compatibility ?

Right, root must be on MST, it works now but not fully...

When MST switch sends priority to a PVST switch it sends default priority
32768, WITHOUT 12 bits extended system id. This is even the case if MST
switch is configured as root. It will still send 32768 to a PVST switch.
This is good because PVST switch will always see an MST switch with lower
priority than itself or other PVST switches. This is assuming that PVST
switches do not have priority configured manually.

But...
in this topology:

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

SW1 and SW2 will both send priority 32768 to SW3 and SW4. As a result SW3
and SW4 will have to compare MAC addresses to determine the root bridge. In
my case PVST chooses SW1 is the root for all vlans. Load balancing is then
impossible.

Is my logic OK ?

On Tue, Mar 1, 2011 at 3:45 PM, Rares Donca <rares.donca_at_gmail.com> wrote:

> Hello,
>
> The spanning-tree root for all Vlans has to be in MSTP, for that to work.
>
> I recommend you to read these excellent blog posts who clearly explain what
>
> is happening there.
>
> http://blog.ine.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/
> http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/
>
> Regards,
> Rares
>
> On Tue, Mar 1, 2011 at 10:33 PM, Jacek <q.192.168.1.0_at_gmail.com> wrote:
>
>> 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.
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
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Received on Tue Mar 01 2011 - 17:45:22 ART