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Adding instances to mst

rimeligraket
Beginner
Beginner

Hi everyone,

My question is very basic - if I add another instance to my mst configuration, will that trigger a reconvergence of mst?

The new instances will now consist of any VLANs already in production or part of another instance.

1 Reply 1

daniel.dib
Rising star
Rising star

MST works by sending a MD5 digest which is used to compare the MST configuration. Name, revision and instance to VLAN mapping makes up this digest whic his compared between all switches.

Adding another instance or moving a VLAN between instances will trigger Topology Change. When the change is done for a while there will be different MST regions because they don't agree on the mapping. The switch with lowest priority for MST0 will be the CIST root. Switches in different regions will have a single port elected as root towards that region, it will be the switch with lowest external cost towards the CIST root. Other ports will be blocked.

I would suggest to only do these kind of changes outside of production hours. Also see this thread which contains a lot of useful information:

https://supportforums.cisco.com/thread/2118116

Daniel Dib

CCIE #37149

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Daniel Dib
CCIE #37149
CCDE #20160011

Please rate helpful posts.