350-401 LACP Port-Channel Practice Question
An engineer is troubleshooting intermittent connectivity issues between two data center switches. The link is a 10GE LACP port-channel. Which misconfiguration could cause packet loss?
⚠ Common exam trap
The trap is that many candidates assume active/passive LACP is a misconfiguration, but it is actually valid. Instead, MTU mismatches are a common source of packet loss in port-channels. Always verify MTU settings when troubleshooting intermittent connectivity.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU size is set to 1500 on one switch and 9000 on the other.
An MTU mismatch between two switches in a port-channel can cause packet fragmentation or drops. With LACP, both sides must have matching MTU settings to ensure proper frame forwarding. If one switch has an MTU of 1500 and the other 9000 (jumbo frames), packets exceeding 1500 bytes will be dropped by the switch with the smaller MTU, causing intermittent packet loss. In contrast, an active/passive LACP configuration is valid and commonly used; it does not cause packet loss if both sides are properly configured. Passive-passive would prevent the port-channel from forming, but that is not the case here.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
MTU size is set to 1500 on one switch and 9000 on the other.
Why this is correct
Mismatched MTU sizes break the link's ability to carry full-size frames consistently. A 9000-byte jumbo frame sent toward the 1500-byte interface is dropped or fragmented, causing intermittent packet loss on the LACP port-channel, especially for large transfers.
- ✗
Auto-negotiation is disabled on both ends.
Why it's wrong here
Disabling auto-negotiation on both ends is consistent and yields a stable 10GE link, so it cannot cause intermittent loss. It is tempting because duplex or speed mismatches do cause loss, but those arise from one-sided settings, not matched manual configuration.
- ✗
Spanning-tree BPDU guard is enabled on the port-channel.
Why it's wrong here
BPDU guard err-disables a port receiving BPDUs, but an LACP port-channel between switches carries no BPDUs on member links, so it cannot cause intermittent loss here. It is tempting because BPDU guard genuinely prevents loops when edge ports face rogue switches.
- ✗
One switch is configured with active LACP and the other with passive LACP.
Why it's wrong here
Active/passive LACP still forms a working port-channel, since passive waits for the peer's initiation. It is tempting because mismatched modes look like a negotiation fault, yet this pairing is a supported design; the failure would require both ends passive, leaving the bundle down.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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