Re: MSTP & PVST+ interoperation

Contrary to popular opinion, this is possible! You can have the Root
bridge on the PVST+ side( I still don't recommend it though ).

In order for this to work, you must configure the STP priorities in
the following manner:

A. Priority for VLAN 1 on the PVST+ switch should be lower than IST0
on the MSTP side.
B. All other VLANS on the PVST+ Root should have a priority lower than vlan 1.

Example:

PVST+ Switch:

VLAN 1 Priority: 8192
VLAN 2-100: Priority 4096

MSTP Switch:

IST0 Priority: 16384

To summarize. MSTP must have a consistent view of the state of its
PVST boundary port. Due to the nature of extend system-id, the
priority values that the MSTP switch sees are inconsistent, and since
IST0 is replicated on all other vlans (at the boundary) the MST switch
thinks the port is ROOT on vlan 1, but DESIGNATED on all of the other
vlans(since the priority on vlan 1 was lower!) So, you must make the
MSTP switch believe the port, and all vlans it receives BPDU's from
are better. (Another solution is to disable extend system-id, but that
isn't possible on many platforms).

I had the same question a while ago. Excellent explanation is here. (
scroll to the bottom )
https://supportforums.cisco.com/thread/163062

-Yuri

On Wed, Sep 26, 2012 at 5:00 AM, Carlos G Mendioroz <tron_at_huapi.ba.ar> wrote:
> After some more thinking ... if the merging code agrees to a vlan to be used
> as "designed" CST on the PVST side, then it would work. I guess that is the
> Brocade master vlan. Nice to know, and an interesting thing if cisco does
> not support something on the line.
>
> -Carlos
>
> Tony Singh @ 26/09/2012 08:17 -0300 dixit:
>
>> Not sure of the solutions for this but afaik you would be breaking stp
>> rules hence port inconsistent state!
>>
>> Schedule an outage and migrate to mst.
>>
>> --
>> BR
>>
>> Tony
>>
>> Sent from my iPhone on 3
>>
>> On 26 Sep 2012, at 11:04, Viet-Hung TON <vton_at_integra.fr> wrote:
>>
>>> Hi Carlos,
>>>
>>> In the boundary switch between the MSTP and PVST, as this router see the
>>> *ROOT* is actually not in the MSTP domain, this device push the
>>> interface in an inconsistency state *BKN* althoudh they see the root
>>> bridge through this port. This case is really like the scenario 3:
>>> *PVST+ and MSTP interoperation* in the blog:
>>> http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/
>>>
>>> But the problem we have is that we cannot change the Root bridge to the
>>> domain of MSTP as mentions in the example.
>>>
>>> That's why I search for a solution which is like the VLAN master in
>>> topology-group of Brocade that permit resolving this case.
>>>
>>> Thanks,
>>>
>>> Viet
>>>
>>> On 09/26/2012 11:46 AM, Carlos G Mendioroz wrote:
>>>>
>>>> Why do you say "the MST region wants to have the Root Bridge inside" ?
>>>> As you say you control the MST area, this "wants" relates to which
>>>> entity ? MST has internal regional root and "global" root bridges.
>>>>
>>>> AFAIK, cisco's MST implementation knows how to merge into a PVST area.
>>>> And that is reflected by the "Bound(PVST)" label (region boundary,
>>>> PVST).
>>>>
>>>> Are your areas multiply connected ?
>>>>
>>>> -Carlos
>>>>
>>>> Viet-Hung Ton @ 25/09/2012 21:01 -0300 dixit:
>>>>>
>>>>> Hi,
>>>>>
>>>>> I have a region of MSTP that must communicate with a zone of PVST.
>>>>> Because of historical reason, I must leave the Root bridge in the
>>>>> PVST+ zone, but in theory, the MSTP region want to have the Root
>>>>> Bridge inside, that's why interfaces of routers in this region are in
>>>>> the unfunctional state when doing a "show spanning-tree":
>>>>>
>>>>> Root BKN* 20000 128.1 P2p Bound(PVST) *PVST
>>>>>
>>>>> Do you have any ideas how to resolve this problem considering that I
>>>>> can just change the configuration in the MSTP zone but not to PVST
>>>>> zones?
>>>>>
>>>>> Is there any things of Cisco like Topology-group with the Master Vlan
>>>>> of Brocade, where we can take a Vlan in mode PVST and mapping all
>>>>> others Vlans to this Vlan (only one instance of PVST)?
>>>>>
>>>>> Thanks for your help,
>>>>>
>>>>> Viet
>>>>>
>>>>>
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>>
>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>
>
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Received on Wed Sep 26 2012 - 11:36:17 ART