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What is a recommended way (in terms of maintenance, simplicity and failsafe-ness) to essentially run a "ring" of switches, but allowing OSPF to run properly (so equal-cost-path routing can work - in picture, for SW4)?

My setup is as follows:

  • Each site has a switch (to which each inter-site link is connected - fiber or wireless)
  • Each site has a router that is only connected via a single interface (doing routing in VLANs)
  • Each site has a VLAN group on the switch to which CPEs connect, and that VLAN is then routed by the router
  • each router runs OSPF to route between subnets

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I have tried running MSTP (with a separate VLAN per link), but I don't seem to get it properly to handle the point to point links, and all common examples on the internet is "enterprise" type - run all the VLANs on all the switches.

Is there some other protocol that would suit the setup better? Is there a fundamental design flaw in my thinking? With MSTP, can different switches have different VLANs or is the protocol fundamentally for running all the VLANs over all the switches in a region?

Thanks in advance.

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  • 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 can post and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 17, 2020 at 17:24

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If you have a fully switched ring MSTP/RSTP is required to avoid the otherwise resulting bridge loop. xSTP blocks one of the links, so it can't be used any more. The routers (L3) can only use what the switches (L2) allow them to.

You have two basic choices:

  1. (best) Migrate to routed links. Routed links can be all activate without a problem. Additionally, you could even load balance (equal-cost multi-path routing ECMP).
  2. (2nd best) Split your VLAN groups into multiple MSTP instances. Each instance has its own root and creates its own spanning tree, so you can arrange those to have all your links active.

I think you've already tried 1. The trick is to either not use spanning tree if there can't be a bridge loop anyway, or to use multiple instances to make them independent.

Additionally, there's shortest path bridging SPB (IEEE 802.1aq) that removes STP's limitations and allows active L2 meshing. However, SPB isn't widely available (yet).

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  • Would I be correct in saying that for routed links I would have to plug each inter-site link into a separate router port? The setup was like that but due to the increase in link number, PoE routers ran out of ports. I have tried to setup #2, but had no success (probably due to my understanding. With MSTP, should all the switches in the region have all the VLANs listed (as in allowed on the inter-site ports - I know that the MSTI should be the same region-wide)?
    – JacquesL
    Commented May 17, 2020 at 11:36
  • If you run out of router ports you can use switch ports as fanout - use distinct VLANs so the links are not bridged, and either no STP or separate instances. MSTP allows for multiple regions and multiple instances in each region. Either can be used to separate the trees, you just need to make up your mind where future expansion is more likely. If there's a somewhat complex structure in each location, use different regions in the ring.
    – Zac67 ♦
    Commented May 17, 2020 at 12:01

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