Cisco CCNA (200-301)Network AccessMedium
A network engineer is troubleshooting an EtherChannel between two Catalyst switches, SW1 and SW2. The engineer suspects an issue with the LACP negotiation. On SW1, interfaces Gi0/1 and Gi0/2 are configured with 'channel-group 1 mode active'. On SW2, interfaces Gi0/1 and Gi0/2 are configured with 'channel-group 1 mode desirable'. What is the expected outcome of this configuration?
- AThe EtherChannel will form successfully, but only one link will be used for traffic.
- BThe EtherChannel will form successfully, utilizing both links.
- CThe EtherChannel will form, but only if both switches are configured for PAgP.
- DThe EtherChannel will not form because of mismatched LACP modes.
Show answer & explanationAnswer & explanation
Correct answer: D. The EtherChannel will not form because of mismatched LACP modes.
LACP 'active' mode actively initiates negotiation, while 'desirable' mode is a PAgP (Port Aggregation Protocol) mode. LACP and PAgP are incompatible and cannot form an EtherChannel with each other.
Why the other options are wrong
- A. Incorrect. LACP and PAgP are different protocols and cannot negotiate to form an EtherChannel.
- B. Incorrect. The EtherChannel will not form due to the protocol mismatch.
- C. Incorrect. 'desirable' is a PAgP mode, but 'active' is LACP. They cannot negotiate.
LACP vs PAgP Modes
LACP (Link Aggregation Control Protocol) and PAgP (Port Aggregation Protocol) are two distinct protocols for EtherChannel negotiation, each with its own active and passive/desirable modes.
- LACP uses 'active' and 'passive'.
- PAgP uses 'desirable' and 'auto'.
- LACP and PAgP are NOT interoperable.
Memory trick: Always Connect Links Properly or Perish Gracefully.