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MST vs. Layer 3 Segregation?

I must be missing the point or something. Why would you move to Multiple Spanning Tree instead of dividing the network at layer 3? As I understand it, the purpose of MST is when you have so many L2 connections, and so many vlans that it becomes necessary to aggregate your instances of STP, and segregate STP domains to prevent unnecessary re-convergence during remote topology changes.

Are there really networks with L2 domains big enough to needs this? Wouldn't an L2 domain large enough to need this already be inefficient?

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level 1
3 points · 2 months ago

Are there really networks with L2 domains big enough to needs this?

Yup, however most designs nowadays eliminate STP almost entirely, so it's likely you won't run into a proper "large" implementation of MST or RPVST. If you do, hopefully they're ready to spend money on an upgrade.

level 1

I've done it when I had a large number of vlans to reduce load on the core switch. There wasn't a large number of hosts but due to segmentation there was a large number of networks.

level 2
Original Poster1 point · 2 months ago

Gotcha, that makes sense.

level 2

Also back in the day of using hsrp with primary and secondary 50/50 split between two cores you'd use MST to keep the root switch as the primary hsrp most the time.

level 2

This is the use case I understood it to solve, tons of vlans with only a couple of spanning tree topologies needed. Makes for easier management for the admin and the switch

level 1

I worked in an MSP and we used MST. Our core network would be shared by a large number of clients, all who came with a number of vlans implemented.

We easily had over 500 vlans

level 1

I was always under the impression MST was developed with carriers in mind. The customers each run their little domain, but the carrier must have something that supports these myriad of technologies. I'm not sure it was ever put into practice this way though.

But, that said, sadly, yes, there are networks like this. I have run into some fairly large campus type networks (10's of thousands of users) and only two L3 devices. Campus was split into separate domains. I thought it was madness, but I was just hired to install it, not take the call when it collapsed.

Aside from that, I love MST because I work in lots of multi-vendor environments and it's the only thing everyone can agree on.

level 1

Yes, only reason I can think of is vendor neutral implementation since mst is an open standard while rpvst is not.

level 1

Why move to MST over layer 3? I don't think any sane engineer ever would. I suspect mst with regions and a big CIST is only crazy enough to be seen on a Cisco exam.

level 2
Original Poster1 point · 2 months ago

Hahaha, yeah, it seemed really weird. Brian Mcgahan basically said the same thing in the video I was watching, but he never really followed up with when you WOULD use it.

Thanks everyone for your feed back.

level 1

I migrated one of our DCs to MST from RPVST because our nexus 9372s could only do 507 vlans in RPVST mode.

But that's being ditched for L3LS VXLAN/EVPN soon.

level 1

Great questions. A lot of time layer 8, high level management, decisions override any good layer 1-7 technical decisions.

In my environment we use MSTP and it works well. We cannot have a L3 access layer since traffic must be inspected by an IPS/IDS. Those systems only live at the distribution layer.

Technically you can run MSTP with a smaller network. PVST+ works but only with Cisco gear. PVST+ does not scale well in any case (50 VLANs is 50 times the BPDUs). With MSTP you can define how many instances and regions you want and only IST, instance 0, sends BPDUs.

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