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?
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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: