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MST clarification

Hi all

What happens if a configured VLAN is not on any MST instance. I understand it would end up in MST 0. Does this mean RSTP would not apply to this VLAN?

TIA

Sam

1 ACCEPTED SOLUTION

Accepted Solutions

Re: MST clarification

Hi Sam,

The BPDU transmitted include an MD5 checksum of the vlan to instance mapping. The switch with a configuration mistake will thus (hopefully) have a different MD5 and its BPDUs will clearly be identify by its neighbors as being part of a different region.

In the very unlikely case where the MD5 digest would match with a different vlan to instance mapping, the consequence could be catastrophic. As we discussed earlier, MST does its calculation on instances only, entirely independently of the vlans. Suppose that you have two bridges A and B connected via two links 1 and 2. Instance 1 blocks link 1 on bridge A while instance 2 blocks link 2 on bridge B. There is no loop because each topology (each instance) has a blocked port. Now, suppose that vlan 1 is mapped to instance 1 on bridge A but mapped to instance 2 on bridge B: this is a permanent loop.

On a port that is detected as being at the boundary of two regions, all the vlans are mapped to a unique instance, the CIST = IST = CIST = instance 0 (all of these are different naming of the same instance!). This way, as long as boundaries are correctly identified, there is no mapping error possible.

The only white paper I know of is in fact mine;-), it was referred to by Ryan in the first answer to your question. Of course, it does not go deep enough in every part, but there is material to write a book on MST, not just a white paper;-)

Regards,

Francois

6 REPLIES
Bronze

Re: MST clarification

Sam

Good question, I didn't know the answer to your question so I did a little research. I understand your question as "Do VLANs that reside in instance 0 (termed IST 0) run RSTP?" I would say yes it does run RSTP.

The IST instance is simply an RSTP instance that extends the CST (for 802.1q networks) inside the MST region.

RSTP is interoperable with 802.1d STP. If the edge of your MST region is running a 802.1d, RSTP will identify this an forward the proper BPDU.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml#ist_instances

Ryan

Re: MST clarification

Yes it would. I think your confusion comes from your previous extended use of PVST (like most Cisco engineers;-)). MST does not run per vlan. MST computes several topologies, one per instance, completely independently of the vlans. Then the vlan to instance mapping assign vlans these topologies.

By default, as you mentioned, all vlans are mapped to instance 0. This means that they will follow the (hopefully fast) transitions computed for instance 0.

Regards,

Francois

Re: MST clarification

many thanks both for the valuable feedback !

Allow me to spice it up a bit ?

What would happen if I configure a vlan as part of MST 2 in all switches apart from one where by mistake it is not in any instance. I can understand(vaguley) it might cause a split in the region. But how would that be reflected ?

Any good ISBNs or links on the subject would be appreciated. I have fixed all my RSTP issues with the help of Francois, but this topic is keeping me interested.

Regards

Sam

Hall of Fame Super Silver

Re: MST clarification

I would strongly suggest to make the MST configuration consistant in the campus. I ran into major issues when I forgot to add one VLAN to the correct MST instance on one switch. The STP was confused and every switch was seeing differnt root bridges, etc. It would result in periodic STP re-convergace. The issue when away when I found the misconfiguration and added the VLAN to the instance.

Hope this helps.

Chris

Re: MST clarification

Hi Sam,

The BPDU transmitted include an MD5 checksum of the vlan to instance mapping. The switch with a configuration mistake will thus (hopefully) have a different MD5 and its BPDUs will clearly be identify by its neighbors as being part of a different region.

In the very unlikely case where the MD5 digest would match with a different vlan to instance mapping, the consequence could be catastrophic. As we discussed earlier, MST does its calculation on instances only, entirely independently of the vlans. Suppose that you have two bridges A and B connected via two links 1 and 2. Instance 1 blocks link 1 on bridge A while instance 2 blocks link 2 on bridge B. There is no loop because each topology (each instance) has a blocked port. Now, suppose that vlan 1 is mapped to instance 1 on bridge A but mapped to instance 2 on bridge B: this is a permanent loop.

On a port that is detected as being at the boundary of two regions, all the vlans are mapped to a unique instance, the CIST = IST = CIST = instance 0 (all of these are different naming of the same instance!). This way, as long as boundaries are correctly identified, there is no mapping error possible.

The only white paper I know of is in fact mine;-), it was referred to by Ryan in the first answer to your question. Of course, it does not go deep enough in every part, but there is material to write a book on MST, not just a white paper;-)

Regards,

Francois

Re: MST clarification

Many thanks again Francois !

I must admit I have learnt a fair bit thru this and previous dialogues on the subject.

But it would be great if u could Write a book and put me out of my misery :-)

Sam

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