Re: MSTP backward compatibility ?

Thanks for correcting me on this. Load balancing will have to be then set
separately for MST switches using bridge id and for PVST using cost or port
priority ?

On Wed, Mar 2, 2011 at 1:51 AM, Petr Lapukhov
<petr_at_internetworkexpert.com>wrote:

> Hi Jacek,
>
> Not sure why you concluded that load-balancing is impossible in this case.
> From the viewpoint of PVST+ side, the MSTP CIST Root appears to be a root
> bridge for every VLAN in the topology. By adjusting STP costs on the links
> connecting PVST+ domain to MSTP domain you can select elect root ports on
> per-VLAN basis and provide limited form of load-balancing just as usual with
> PVST+ scenarios.
>
> --
> Petr Lapukhov, petr_at_INE.com
> CCIE #16379 (R&S/Security/SP/Voice)
> CCDE #20100007
>
> Internetwork Expert, Inc.
> http://www.INE.com <http://www.ine.com/>
> Toll Free: 877-224-8987
> Outside US: 775-826-4344
>
> 2011/3/1 Jacek <q.192.168.1.0_at_gmail.com>
>
> 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.
>> >>
>> >>
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>> >>
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Received on Wed Mar 02 2011 - 23:13:42 ART