question about MST

Answered Question

Hi !

I'm currently preparing my CCNP certification.

I need some clarification about my reading.

1- on my reading I had understand if it's possible to avoid to have data trafic in the instance 0 in MST it's better for avoiding problem.

2- Is it same kind of BPDU inside a MST region than the outside ? if yes what's difference between them.

thanks a lot for your help !

I have this problem too.
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Correct Answer by Francois Tallet about 8 years 10 months ago

Hi Christian,

1- In the IEEE standard, all MST instances exist on every port. In order to provide a behavior that is closer to PVST, we only send the MST information for a particular instance on a port when there is at least one vlan mapped to this instance configured on the port. However, we could not do this trick for the CIST (instance 0) that definitely has to be present on every port in MST. The recommendation is just a simple way of avoiding this problem. I'm not sure we were right in doing this recommendation because many people consider it as a rule. It is definitely not. Mapping vlans to the CIST is perfectly legit, and in particular, if you are using a single instance in your network, you have no reason to use something different than instance 0.

A brief summary of the reason for the recommendation is at:

http://www.cisco.com/warp/public/473/147.html#ist_active_ports

2- Yes, it's the same BPDU. Bridges that don't understand MST (like RSTP bridges) or that are in a different region just read the CIST information from the MST BPDU. They ignore the part that is specific to MSTIs.

HTH,

Francois

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Correct Answer
Francois Tallet Mon, 02/11/2008 - 10:35

Hi Christian,

1- In the IEEE standard, all MST instances exist on every port. In order to provide a behavior that is closer to PVST, we only send the MST information for a particular instance on a port when there is at least one vlan mapped to this instance configured on the port. However, we could not do this trick for the CIST (instance 0) that definitely has to be present on every port in MST. The recommendation is just a simple way of avoiding this problem. I'm not sure we were right in doing this recommendation because many people consider it as a rule. It is definitely not. Mapping vlans to the CIST is perfectly legit, and in particular, if you are using a single instance in your network, you have no reason to use something different than instance 0.

A brief summary of the reason for the recommendation is at:

http://www.cisco.com/warp/public/473/147.html#ist_active_ports

2- Yes, it's the same BPDU. Bridges that don't understand MST (like RSTP bridges) or that are in a different region just read the CIST information from the MST BPDU. They ignore the part that is specific to MSTIs.

HTH,

Francois

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