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I'm trying to come up with a plan to get our core network design a bit more resilient to failure.

Here is a drawing of what I would like to achieve: enter image description here

The final design would probably involve one more instance, but the drawing got way too cluttered with a third line all around the place.

What I've figured out on my own is how to set the MST instance root for each instance. What I don't quite get is how I'm supposed to design this that RACKSW02 takes over root for MST2 if RACKSW01 dies etc. Is it just a matter of setting the correct MST root priorities around the setup?

Do I need to set custom link costs if I want to override the default metrics, as RACKSW01<->RACKSW02 is 20gbps, while CORESW01/02 only has 10gbps?

The design goal behind all this is to balance out the load across the links, as 99.9% of the MST2 instance is only between the two top-of-rack switches.

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 9 '17 at 18:28
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Yes, it's that simple, just set priorities appropriately :-) So you configure your Core 1 with highest and Core 2 with lower priority for instance 1, and configure instance 2 similar way. You leave other devices in the network to their default priorities. The only way to overwrite the default link costs is to set them manually.

I would consider a different configuration. There's no point setting your STP root to the edge. Rather leave the core switches as roots for different MSTP instances; for example Core 1 for odd and Core 2 for even instances. This way the root ports are core uplinks so non-blocking, and you don't have to care about the link costs on the edge. Another advantage is that it's easier to keep track on which ports are blocking on which instances.

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