Cisco Certified Support Technician (CCST) NetworkingNetwork AccessHard
A network engineer is configuring an EtherChannel (Link Aggregation Control Protocol - LACP) between two switches. The engineer has configured the port-channel interface, but the physical interfaces are not bundling. Upon review, the engineer finds that one switch port is configured for 'speed 100' and the other for 'speed auto'. Which of the following is the MOST likely reason for the EtherChannel not forming?
- AMismatched Duplex settings.
- BMismatched Speed settings.
- CPhysical cables are not connected properly.
- DIncorrect LACP mode (e.g., active/passive).
Show answer & explanationAnswer & explanation
Correct answer: B. Mismatched Speed settings.
For EtherChannel to form successfully, all participating physical interfaces must have identical configurations, including speed, duplex, native VLAN, and allowed VLANs. Mismatched speed settings, such as one port set to a fixed speed and the other to auto-negotiate, will prevent the EtherChannel from bundling.
Why the other options are wrong
- A. Mismatched duplex settings would also prevent EtherChannel, but the scenario explicitly states mismatched speed configurations as the primary difference.
- C. Physical cables not connected properly would result in no link at all, which is a more fundamental issue than mismatched speed settings on active links.
- D. Incorrect LACP mode might prevent LACP from forming, but the fundamental issue of mismatched speed would still prevent the physical interfaces from being considered compatible for bundling.
EtherChannel Configuration Requirements
EtherChannel (Link Aggregation) bundles multiple physical Ethernet links into a single logical link, but requires identical configuration parameters on all bundled interfaces.
- All ports in an EtherChannel must have the same speed and duplex settings.
- All ports must be in the same VLAN or configured as trunk ports with identical trunking parameters.
- LACP (802.3ad) and PAgP are common negotiation protocols.
Memory trick: Identical Settings, Happy Bundles