The answer is that the Ethernet interface MTU is set to 1500 instead of 2500, which causes fragmentation and poor FCoE performance. Fibre Channel over Ethernet encapsulates native 2148-byte Fibre Channel frames into Ethernet frames, requiring a jumbo MTU of at least 2500 bytes to avoid fragmentation. When the MTU is left at the default 1500, the FCoE frames are broken into smaller packets, leading to excessive reassembly, retransmissions, and severe performance degradation—even though connectivity appears to work intermittently. On the Cisco DCCOR 350-601 exam, this scenario tests your understanding of FCoE encapsulation overhead and the critical role of MTU tuning in converged networks. A common trap is assuming that successful login means the configuration is correct, but FCoE can limp along with a 1500 MTU, just poorly. Remember the memory tip: “FCoE needs 2500—no fragmentation, no frustration.”
350-601 Storage Network Practice Question
This 350-601 practice question tests your understanding of storage network. 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 deploying FCoE on a Cisco Nexus 9000v switch in a converged network. The storage array is connected via native Fibre Channel to an MDS switch, and the MDS is connected to the Nexus using an FCoE link. The engineer creates a virtual Fibre Channel (VFC) interface on the Nexus, binds it to an Ethernet interface, and maps VSAN 200 to VLAN 200. The MDS side has an FCoE port configured and enabled. Servers connected to the Nexus with FCoE initiators can successfully log into the storage targets, but performance is very poor and intermittent. The engineer checks for drops on all interfaces and finds none. The engineer also verifies that the FCoE VLAN is not blocked by spanning tree. What is the most likely cause of the performance issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Ethernet interface MTU is set to 1500 instead of 2500.
FCoE requires a larger MTU (typically 2500 bytes) to encapsulate Fibre Channel frames without fragmentation. An MTU of 1500 causes fragmentation and retransmissions, leading to poor performance. Option B is correct. Option A would prevent login entirely. Option C is not required for basic connectivity. Option D was already verified as not causing the issue.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The Ethernet interface MTU is set to 1500 instead of 2500.
Why this is correct
FCoE requires jumbo frames; 1500 MTU causes fragmentation.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
The FCoE VLAN is blocking spanning tree.
Why it's wrong here
The engineer already verified that spanning tree is not blocking the VLAN.
✗
The MDS has not enabled FCoE on the interface.
Why it's wrong here
Login works, so FCoE is enabled on the MDS.
✗
The VFC interface is not bound to the correct port-channel.
Why it's wrong here
Binding to a port-channel is not necessary for single Ethernet links.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-601 questions on switching, trunking, and access-port configuration.
Storage Network — This question tests Storage Network — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The Ethernet interface MTU is set to 1500 instead of 2500. — FCoE requires a larger MTU (typically 2500 bytes) to encapsulate Fibre Channel frames without fragmentation. An MTU of 1500 causes fragmentation and retransmissions, leading to poor performance. Option B is correct. Option A would prevent login entirely. Option C is not required for basic connectivity. Option D was already verified as not causing the issue.
What should I do if I get this 350-601 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-601 questions on switching, trunking, and access-port configuration.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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