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Imagine a pair of switches connected with a couple of trunk links created in the following fashion:

Two switches connected with a pair of trunks

Assuming the numbers on lines correspond to different VLANs, how would you arrange a working spanning tree configuration so that both of these links are active?

What I am trying to do is to separate certain VLANs over P20 link and the bulk of the remaining to be routed over P21 but so far, all I've been able to do get one of these two to work and the other one to be blocking. What's even worse, I can't clearly predict which one would be blocked.

The switches in question are a Dell 8024F and a Dell N3024 - the former doesn't support RPVST whereas the latter does, so it looks like I am stuck with Rapid Spanning Tree (RST). Is there any way to configure RST to allow routing on two trunks between two switches, or do I need to consider Multiple Spanning Tree (MST) protocol?

What is the way to best accomplish this setup? I would like to escape from complexity of the MST but if that is the only way, I would appreciate some hints/pointers on how to best set this up as I am not a network engineer and MST seems quite complex and finnicky.

P.S. I can share the full topology of my network (which I omitted for brevity) but, in a nutshell, there are a handful of switches arranged in a daisy-chain fashion (i.e. no loops/cycles between switches). Several of these would be having multiple trunks carrying different VLAN traffic.

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Short answer: no.

RSTP is entirely VLAN agnostic, so there's no way to make both trunks forwarding, regardless of which VLANs they use.

You need either RPVST+ or MSTP on both switches, then you can use port priorities to make the trunks active simultaneously.

Your only other option is to deactivate spanning tree on those ports (=filter BPDUs) and be very careful not to create looped VLANs. You might want to configure broadcast rate limits for damage control.

Note that the scheme in your diagram provides no failover anyway, so you're not losing anything. For a failover scheme you'd have to run all VLANs on both trunks and configure MSTP with two instances so that each trunk is only actively used for the indicated VLANs. With just two switches that is most easily accomplished by using different port priorities in both MSTIs.

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  • Thank you - I suspected as much. I am not currently concerned with failover, however. Is there an MSTP primer that you would recommend?
    – quantum
    Mar 21, 2023 at 0:43
  • Also, what kind of interoperability exists (if any) between MST and RST? In particular - if I were to migrate this pair of trunks to MST, do I have to do this on all the other switches in the network for the spanning tree to correctly work?
    – quantum
    Mar 21, 2023 at 0:52
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    Sorry, resource recommendations are explicitly off topic here. MSTP interoperates with RSTP so that they share MSTP's CIST (common and internal spanning tree). RSTP cannot participate in other MSTIs. You could use two new MSTIs for those trunks while leaving the rest of the network at RSTP/CIST (depending on where they're connected - you shouldn't split the CIST).
    – Zac67
    Mar 21, 2023 at 7:01

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