I have a Layer 2 RSTP network and it works fine.

However, the RSTP root is on the other side of a switch which does not participate in the RSTP algorithm – and this switch has two physical connections to the RSTP root.

If the non-participating switch forwards packets out on both physical connections to the RSTP root – how does the RSTP root deal with the duplicates?

Thanks, Matt

  • 5
    Can you post some of your configurations? If you have 2 connections to the root, one goes into Blocking, otherwise you'd have a switching loop.
    – Ryan Foley
    Dec 10, 2013 at 9:55
  • 1
    Can you define your question in more clear way ?
    – rmaan
    Dec 10, 2013 at 10:48
  • 4
    The middle switch does not run STP at all? Is it totally transparent so that BPDUs get forwarded or does it still consume the BPDUs? What kind of devices are you running?
    – Daniel Dib
    Dec 10, 2013 at 11:55
  • 1
    @Matt Can you provide us RSTP states for relevant ports? I think that the second port going from the root into the dumb switch might be in backup-blocking state.
    – AndrejaKo
    Dec 10, 2013 at 21:45
  • 2
    When you say two physical ports, are they set up in a link aggregate? If not, I'd think that at least one of those ports would already be shut down, right? Feb 28, 2014 at 2:54

1 Answer 1


Let's consider only the Root Bridge and the switch not participating in RSTP.

The root bridge (which runs RSTP), puts one of its port connected to the non-RSTP switch, to Backup role (Discarding state) and the other port continues in Designated role (Forwarding state).

Reason: Initially both ports of the Root Bridge sends BPDUs. The peer non-xSTP node, floods the BPDU (doesnot forward BPDU since Dest MAC is a multicast), so the BPDU is received by another port of the ROOT Bridge.

  • Then comparison takes place with the information in BPDU and the information in the receiving port.
  • Here all the parameter matches (Root BridgeID, Root Path Cost,... ) except the PortID parameter.
  • Hence among the port with lower (Better) PortID will be chosen as Designated, and other will be chosen as Backup.

[In some switches, BPDU dest MAC frames, will be dropped, which might lead to loops]

Kindly confirm whether the answer clears your doubt.

  • Welcome to NE, we hope you will both contribute to and learn from this community. Thank you for providing an answer to this old question. Since the OP never provided more details, I think people suspected it was a homework question and failed to answer. After all this time though, if it was homework it is too late, so glad it finally has an answer.
    – YLearn
    Nov 21, 2015 at 18:34

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