Unanswered Question
Sep 20th, 2008

hi every body!

i want to create mst1 and use the following command:

sw1 spanning-tree mst1 priority 200

The above result in mst1 with switch 1 as a root.

1)My question is if switch 1 crashes, will we have to create mst1 again using different switch as root or mst1 will be created and the 2nd lowest bridge priority switch will become root after sw1 crashes and no manual intervention is needed?

2)Can we use extended sysytem id with mst?

3)Does the Command " spanning-tree mst 1 root primary" works the same way as it does in stp?

i.e if the current root priority is above 24,576, then 24,156 will be set as bridge priority on the switch

4)should the timers tuning such as helo timer forward age, max-age be configured on root switch for ist only?

thanks a lot!

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Giuseppe Larosa Sun, 09/21/2008 - 01:17

Hello Sarah,

1) all the MST configurations has to be performed on all switches that take part in the MST region.

the MST BDPU contains a checksum that is calculated by using the info of the vlans to instances association if there is a mismatch the two switches the two switches don't consider themselves as part of the same region.

So when root bridge SW1 for MST instance1 fails a new root bridge is elected.

If later Sw1 becomes operational again it will take over and will be again the root bridge.


in the command reference I didn't found an explicit sentence about this but it is said that priority should be set in multiple of 4096 as happens when extend system-id is active.

Further they say there are 6500 chassis with only 64 Mac addresses where you cannot disable extend system-id.

the range of IST instances number is from 0 to 4094 (but no so many can be active at the same time up to 65 for the standard) so extend system id applies with instance number + base priority instead of vlan number + base priority.

So the answer is that is already used by default with instance number used instead of vlan number.

3) yes and there is also the secondary option


4) in classic STP is enough to change timers settings on the root bridge that will be propagated with its BPDUs here you can change them for all instances (no instance option here) I would expect the same behaviuor here.

Hope to help


Francois Tallet Mon, 09/22/2008 - 08:32

1) I think your question is related to how mstis are "created". An msti is created as soon as a vlan is mapped to it in the mst configuration. It is independent of the instance priority (that can be configured even if the instance is not running). So basically, if the root for mst1 goes away, another is elected within the region using the usual stp procedure (lowest bridge id).

2) you cannot not use it;-) The way mst is designed, you can only set the priority by increment of 4096. All the instances use the same mac address as their bridge id.

3) yes, but as usual I would recommend simply tuning the priority instead.

4) timers are only configured in instance 0. This is because instance 0 (the CIST) is the only one sending BPDUs. The other instances (the MSTIs) entirely depend on the CIST to propagate their information and thus don't use a timer of their own.

If you want, an MST BPDU is an RSTP BPDU representing the CIST, plus some additional records representing the information for the MSTIs appended at the end.




This Discussion