Production VLAN on MST instance 0

[kradjesh13]

Hi,

What will be the implication of using a VLAN on MST instance 0.

I understand it’s a bad practise to use MST instance 0 for a production VLAN but I cannot rationalise and could not find an answer behind and the reasons behind this. Can someone shade some light on this topic please ??

Many Thanks

Rajesh

[Rolf Fischer · Accepted solution]

Hi Rajesh,

I wouldn't say that using instance 0 for production VLAN is generally a bad practice.

There are issues when you have different MST regions or boundaries to STP bridges which are under different administrative control, Instance 0 is always involved in such cases. In MSTP, BPDUs are only transmitted in instance 0, the relevant information of the other instances are contained in supplements called M-records.

An example:

You have a boundary to a RSTP bridge in VLAN 200 which is mapped to instance 2. A topology change comming form that brigde will be forwarded inside your region in instance 2 (M-record) and in instance 0 ("Main-BPDU"). Thus, you'll see CAM-table flushing in VLANs mapped to instance 2 (like expected) but also in VLANs mapped to instance 0 - and this is in most cases not desired.

There's a  very recommandable blog in ine.com:

http://blog.ine.com/2010/02/22/understanding-mstp/

Hope that helps

Rolf

[kradjesh13]

Hi Rolf,

Thanks for the update and i am trying to dig little bit more on this topic.

Thanks

Rajesh

[Jairo Sarmiento Garzon]

Hi Rolf,

Reading your reply, I think that it's useful to map vlans to another instance different to 0 when I have more than two instances (instance 0 and 1), isn't it?

The questions is because i'm currently have a configuration with those two instances, some vlans mapped to instance 0 and others to instance 1, and I have the doubt if It's recommended to change all vlans mapped to instance 0 to instance 1.

Is that necessary? This is my current configuration:

Instance Vlans mapped
-------- ---------------------------------------------------------------------
0           1-39,41-59,61-69,71-109,111-119,121-129,131-139,141-149,151-159
             161-179,181-189,191-199,201-269,271-324,326-599,602-799,802-817
             819-822,824-900,905-917,919-922,924,926-937,939-4094

1           40,60,70,110,120,130,140,150,160,180,190,200,270,325,600-601
             800-801,818,823,901-904,918,923,925,938
-------------------------------------------------------------------------------

What is your opinion?

Thanks.