Clarification on MST

Eildor

I'm reading through the CCIE Routing and Switching OCG, and I'm left with quite a few questions. For example, in what insta... scenario would one want to use multiple MST regions? A practical example would be great.

EdTheLad

You have 4000 vlans in your l2 core, you are using spanning tree, you only have a finite number of paths. Why not group vlans into mst instances and send a finite number of bpdu's rather than one per vlan.

Eildor

This is a great justification for using instances, but I'm more interested in knowing when multiple regions are used in the real world. I haven't really been able to find any documentation on MST region design either.

rinoel

Have a look:

Understanding MSTP

lrb

I've never actually seen any businesses use multiple MSTs regions so I'm at a loss to try and give you a real world example. I guess if you had a few sites connected via an L2 service, with each site having different intra-site switched traffic engineering requirements, and you wanted to minimize the amount of BPDUs tunnelled between the sites, you could run each site as its own MST region and then connect via the CST.

Fitzi

My understanding is that MST can be used by hosting providers for small DC builds to provide multi tennancy for their customers (to get more than 4096 vlans) without going to an MPLS design. I think Kurt Bales mentioned this on one of the packet pushers episodes when they were comparing DC switches, not sure which one it was, though like IRB said, I have never seen this in production either.

Jackace

I have used MST to load balance across two layer 2 links. I work for a Service provider and a hospital in our area ordered a 1 gig link to their remote backup site and then ordered 10+ remote sites to be delivered over another gig link. They then wanted failover on these 2 links for both services. We turned up 2 MST regions and put the vlan for the data center in one and all the remote site vlans in another. We then preferred one MST region on one gig link and the other MST region on the other. I would have rather installed a 10G capable device on site and routed everything, but that wasn't an option for various reasons.

Eildor

Thank you for your input guys