Kb178
Emulab FAQ: Testbed Operations: How do I deal with bad ports and Cisco switch trunking?
We found what appeared to be a bad port and/or wire in the trunk between cisco3 and cisco4. So, we moved the wire to a new port that was not contiguous with the other three, and tried to make a trunk with non-contiguous ports, skipping the bad one.
It seems that this is not legal. Instead of telling me this, the switch just silently made two trunks. So we had a forwarding loop in the switch topology between ciscos 3 and 4.
Instead, we dropped back to 3 gigabits between ciscos 3 and 4 temporarily, instead of 4.