I have the following setup:

Distro Switch 1 <-> Distro Switch 2
           Access Switch

One cable is going from port 47 to Distro switch 1 and another is going from port 48 to Distro switch 2. Vlan 1 is configured for this particular example. There is an LACP trunk setup btw the distro switches to pass vlan traffic.

However, If both 47+48 are enabled on the access switch, I thought one of the ports would go into a blocking state. That is not the case. Both ports stay forwarding. If I do a show spanning-tree vlan 1, I do not see any trunk ports in the vlan on any of the switches. Is there something I'm missing on the trunk configuration for this to work properly? Ultimate goal is for redundancy for the access switch.

Btw, I am running 802.1w.

I am running Foundry SX's for the distro switches and have a cisco 3550 for the access switch.

  • 2
    where is the STP root, and can you confirm that the 3550 really is running 802.1w? Jul 29, 2013 at 18:35
  • 1
    Could you give us the output of show spanning-tree root on the 3550? Jul 29, 2013 at 18:59
  • 1
    Hi, please don't forget to accept the answer that was helpful to you Jul 31, 2013 at 9:54
  • 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.
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    Jan 4, 2021 at 22:38

1 Answer 1


I am assuming several things since there are some points missing. You did not configure anything except for LACP ether-channel between the distro switches. NOTE: LACP is ether-channel protocol not trunking protocol.

You need to check several for several things:

  1. Ports 47 and 48 are trunks (either statically configured or negotiated through DTP) check on both sides the distro switches and cisco side. sh int trunk should list 47 - 48

  2. If the ports are trunk, but the sh spanning tree vlan 1 does not include them make sure that VLAN 1 is allowed on that trunk. If it is not, these ports will not show up in sh spanning vlan 1

Now as for why both ports stay in forwarding, it could be a simple issue of which switch is becoming the root bridge, that is cisco 3550, since both of the ports stay forwarding. You need to configure bridge priorities on distro switches so that one of them becomes root bridge and the other becomes secondary root bridge. This way you will now for sure that cisco 3550 s port 48 will become blocked if the root bridge will be the distro 1 switch and the secondary will be the distro 2 switch.

Also try without the ether-channel to eliminate some possible miss-configuration issues or bugs.

  • 1
    and don't forget that the 3550's default stp protocol is PVST, which would form an STP boundary with any 802.1w switches Jul 29, 2013 at 18:54
  • good point, totally missed that. Jul 29, 2013 at 18:56
  • yeah, turns out the 3550 doesn't support pvrst. Jul 29, 2013 at 19:21
  • #spanning-tree mode ? mst Multiple spanning tree mode pvst Per-Vlan spanning tree mode Jul 29, 2013 at 19:21
  • @NetworkNinja, pvrst is not the same as 802.1w. Cisco MST with everything in one STP instance gets you as close as possible to 802.1w Jul 29, 2013 at 19:35

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