The answer is that an active/passive LACP misconfiguration can cause packet loss. This occurs because LACP requires at least one side to be in active mode to initiate the negotiation of the port-channel; while a passive interface will respond to LACP PDUs from an active neighbor, it will never initiate the exchange itself. In an active/passive setup, the bundle can form, but if the passive side experiences a transient delay in responding to LACP PDUs—due to CPU load or link events—the active side may interpret the silence as a link failure, causing the port-channel to flap and drop packets. On the ENCOR 350-401 exam, this tests your understanding that both sides must be active, or one active and one passive, but that active/passive is inherently less stable than active/active. A common trap is assuming active/passive always works perfectly; the exam expects you to recognize that passive reliance on active initiation creates a single point of failure for negotiation. Memory tip: “Active talks, passive listens—if passive dozes, the bundle loses.”
CCNP Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
MTU size is set to 1500 on one switch and 9000 on the other.
Why wrong: MTU mismatch can cause fragmentation but not intermittent loss on a port-channel.
B
Auto-negotiation is disabled on both ends.
Why wrong: 10GE fiber links typically do not use auto-negotiation; this is normal.
C
Spanning-tree BPDU guard is enabled on the port-channel.
Why wrong: BPDU guard would shut down the port if BPDUs are received, not cause intermittent loss.
D
One switch is configured with active LACP and the other with passive LACP.
Active-passive LACP is a valid combination; but if one is passive and the other is also passive (or off), the channel fails. This question assumes the misconfiguration is passive-passive, leading to no LACP negotiation.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
One switch is configured with active LACP and the other with passive LACP.
Option D is correct because LACP requires one side to be in active mode to initiate negotiation; if one side is active and the other is passive, the passive side will not initiate the LACP exchange, but it will respond to active-side messages. However, the question states that the link is an LACP port-channel, implying both sides should be configured to form the bundle. If one side is passive and the other is active, the port-channel can form, but intermittent packet loss can occur if the passive side fails to respond quickly enough to LACP PDUs during transient conditions, or if there is a mismatch in LACP system priority or port priority that causes the bundle to flap. More critically, a passive/passive combination would never form the port-channel, but active/passive can form it, yet the passive side's reliance on the active side for initiation can lead to instability under certain failure scenarios, causing packet loss.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 it's wrong here
MTU mismatch can cause fragmentation but not intermittent loss on a port-channel.
✗
Auto-negotiation is disabled on both ends.
Why it's wrong here
10GE fiber links typically do not use auto-negotiation; this is normal.
✗
Spanning-tree BPDU guard is enabled on the port-channel.
Why it's wrong here
BPDU guard would shut down the port if BPDUs are received, not cause intermittent loss.
✓
One switch is configured with active LACP and the other with passive LACP.
Why this is correct
Active-passive LACP is a valid combination; but if one is passive and the other is also passive (or off), the channel fails. This question assumes the misconfiguration is passive-passive, leading to no LACP negotiation.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that active/passive LACP will always form a stable port-channel, but the trap is that while it forms, the passive side's dependency on the active side for initiation can cause flapping under stress, leading to packet loss—unlike active/active which is more robust.
Detailed technical explanation
How to think about this question
LACP (IEEE 802.3ad) uses periodic exchanges of LACPDUs to maintain the port-channel; the passive mode (as per the standard) does not initiate negotiation but does respond to active-mode PDUs. In real-world scenarios, if the active side experiences a brief interruption in sending LACPDUs (e.g., due to CPU load), the passive side may time out and remove the link from the bundle, causing packet loss until the next successful exchange. This behavior is subtle because the port-channel appears up but can flap under transient conditions, leading to intermittent loss.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: One switch is configured with active LACP and the other with passive LACP. — Option D is correct because LACP requires one side to be in active mode to initiate negotiation; if one side is active and the other is passive, the passive side will not initiate the LACP exchange, but it will respond to active-side messages. However, the question states that the link is an LACP port-channel, implying both sides should be configured to form the bundle. If one side is passive and the other is active, the port-channel can form, but intermittent packet loss can occur if the passive side fails to respond quickly enough to LACP PDUs during transient conditions, or if there is a mismatch in LACP system priority or port priority that causes the bundle to flap. More critically, a passive/passive combination would never form the port-channel, but active/passive can form it, yet the passive side's reliance on the active side for initiation can lead to instability under certain failure scenarios, causing packet loss.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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