Working on setting up a new set of switches. When one of our offices network was setup we did not change any of the spanning tree settings and just let everything work itself out. We are now expanding and need to work on changing the spanning tree setup a bit.

Below is a diagram of the network switches we are adding. What we are finding is that STP is blocking the LACP connection of the 2x 40GB cables between the switches up to the core.

The question that we are having is how do we configure the ports for the LACP between the 2x Dell N4064 switches so that traffic flows between them before hitting the root bridge core switches? The port cost to get back to the root bridge is currently 2000 and the port cost across the 80G LACP between the switches is 250.

Currently the red X is where the ports are disabled and we would like to have the ports disabled at the purple X.

Root Bridge has a configured spanning-tree priority of 4096. The other two switches both have the same priority of 20480.

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  • Looks like it is a duplicate. I did not see that when initially posting. Thanks for pointing that out.
    – DanielJay
    Oct 3, 2014 at 12:30

1 Answer 1


Increase the port cost on the uplink to the root switch to something greater than 3500 on either N4064 switch. That will cause that port to block and the LACP bundle to start forwarding.

  • See your previous answer to the same problem. The issue is the selection of root bridge.
    – Ricky
    Oct 2, 2014 at 22:16

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