MSTP BLK PORT

[CSCO12212629]

Hello guys,

I have a problem understanding and configuring MSTP. Need your help.

In this scenario

All the switches have that stp mst mode configure.

Except that all the access switches don't have instance 3.

For instance 0-1, the access sw1 port 47 will be blocked and the distribution 2 will become root bridge (which is correct i suppose)

When come to instance 2-3, the access switch 1 will elect itself as root bridge

and will be block the same port (gi1/0/47) for all instances.

We have tried to enable debug and clear the stp protocols, configure name and revision number but the problem still the same.

Is there any config missing, or its the structure that is causing the problem?

Please kindly advise. Thanks!

[Jan Hrnko · Accepted solution]

Hi,

All the switches have that stp mst mode configure.
Except that all the access switches don't have instance 3.

I am afraid that your switches are not really running in the same MST region. The thing is, that ALL of the switches in the SAME region need to have:

• Identical NAME of the region
• Identical REVISION NUMBER
• Identical list of INSTANCES and VLANs assigned to them

Try to configure ALL switches with SAME: name, number and mapping - thus creating a common MST region.

You can't just exclude instance 3 from access switches.

That should resolve the problem, as now it seems that the switches think and act like they are in different MST regions.

Best regards,

Jan

[CSCO12212629]

Hi Jan,

Thank you for your advice.

After adding instance 3 for the access switch i am able to get the expected result.

best Regards,

Apie >.<