(Ring Network A)----[Router A]-------(Possibly Ring Network C)-------[Router B]----(Ring Network B)
- 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.
- Several questions were asked for if it's actually a router, and what device is it. It's a layer 3 switch/router. RuggedCom RX1512
- Based on some of the comments and answers, it sounds like the RSTP BPDUs from one network should not be getting through the router and influencing the other networks. Comment from Ricky states: "Routers don't participate in STP. (switch modules in a router may)". This may be my situation.
Quick side note: as mentioned, it's a L3 switch, and I'll keep using the word 'router' to be consistent.
I am using SVIs. There's a VLAN and IP applied to interfaces in Network A, and similarly to the Network C facing interfaces. Different subnets. No tagging or anything going on. Only using VLAN and SVIs to define the network separation.
This seems to say that STP would not propogate between SVIs. Right?
It switches between ports (within the same VLAN), and routes between VLANs (using SVIs) or routed ports. STP is relevant for the former but irrelevant to the latter, which I was trying to point out above.
But this seems to say that it would cross SVIs.
However, switching ports all participate in a single spanning tree (for RSTP), and you need to configure them as you want things to work. You should note that RSTP is VLAN-agnostic and forms a single spanning tree. It works purely on the port level, as if you had an entirely 'flat' network.
I very much agree with this, and I think that's what I'm trying to do.
In your case, you should split the STP domains.
Shouldn't SVIs split the STP domains? That's what I thought would happen. If this is supposed to occur, I must have something incorrectly configured or the router is not being compliant (doubtful).