Cisco – Why doesn’t the port-channel interface come up, between Cisco Catalyst switches

ciscocisco-catalystetherchannel

To connect two Catalyst switch stacks, I use EtherChannel, i.e. I aggregate several ports on each switch to get more bandwidth and higher availability than with just a single port.

However, the link doesn't work yet. What could cause the problem?

Best Answer

Improperly configured EtherChannel (port-channel) interfaces would be disabled automatically, to prevent problems such as loops. First of all, having the same configuration on each side would be the safest.

Here's a checklist:

  • Both switches must see the ports as an EtherChannel bundle. If one switch would consider ports as separate connections, it's inconsistent and would fail.

  • All ports in the bundle needs to have the same EtherChannel protocol, i.e. don't mix PAgP and LACP. Also manual configuration mixed with one protocol is not recommended.

  • All channel ports must use at the same speed and the same duplex mode. And LACP doesn't support half-duplex.

  • No one of the ports can be SPAN (switched port analyzer) destination port.

  • If the channel is layer 3, give the address to the port-channel interface, not to single ports.

  • For layer 2 channels:

    • All ports have to be either trunks or in the same VLAN.
    • If trunking is used, the mode has to be the same on all ports.
    • If you use a trunk with an allowed VLAN range, that range must be the same on all ports.
    • If you would have different STP costs in a channel, at least ports have to be compatible to each other. STP considers an EtherChannel as a single port.
    • Protocol filtering has to be the same on all ports.
    • Also here, (static MAC) addresses must be on the port-channel interface, not on single ports.

Further reading:

Related Topic