router – RSTP Separation – Network Engineering Stack Exchange

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Topology:

(Ring Network A)—-[Router A]——-(Possibly Ring Network C)——-[Router B]—-(Ring Network B)

Goals

  • I need Router A to always be the RSTP root bridge for Network A.
  • I need Router B to always be the RSTP root bridge for Network B.
  • Without getting into details, Network C doesn’t always exist; or at
    least the link to A and B doesn’t always exist.
  • I don’t care what role Router A and B take within Network C.

I currently have this configured for RSTP. But what happens is that when Network C connects Router A to Router B, TCNs happen and devices in Network A, and Router A, will declare Router B as the root bridge (it has lowest MAC). Since Network C comes and goes, it causes interruptions during reconfiguration/convergence.

So what is the proper solution here? Some ideas:

  • Turning off Spanning Tree on Network C facing ports on Routers A and B
  • MST… I have never used it, as it seems to be a way to combine various VLANs into a single spanning tree instance.. and that’s not really what I’m trying to do here. It does seem to support the concept of “regions” or “boundaries” though, and that may be what I need.

Some additional notes that may add clarity.. or may muddy the waters:

  • I stated that network C is “possibly” ring. It is not a ring right now, but may be in the future.
  • I hadn’t realized I would need to do something like this because I (incorrectly) assumed that a routable interface would not forward BPDUs from a routable interface in another network. I essentially thought the RSTP domain would stay within the layer 2 network. i.e. Router A would be the root bridge on the interfaces facing Network A, and perhaps non-root (designated bridge?) on the interface(s) for Network C.

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