Question 956 of 984
FCoE PFC Troubleshooting for Intermittent Timeouts
During an FCoE deployment, the server team reports that hosts can reach the storage array but performance is intermittent with periodic timeouts. The network team sees no errors on the FCoE VLAN. The DCB configuration on the upstream switch shows that PFC is enabled for CoS 3. What should the engineer check next?
Quick Answer
The correct next step is to check that the PFC configuration matches on both ends and that PFC is enabled on the FCoE VLAN interfaces. This is because Priority Flow Control must be consistently applied on every hop of an FCoE link to guarantee lossless behavior for storage traffic; if PFC is enabled for CoS 3 on the upstream switch but not on the server’s vNIC or the FCoE VLAN interfaces, the lack of pause frames will cause intermittent frame drops and periodic timeouts, even when the FCoE VLAN itself shows no errors. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this scenario tests your understanding that FCoE relies on a lossless fabric—PFC misconfiguration is a classic trap where engineers focus on VLAN errors instead of verifying end-to-end DCB consistency. A useful memory tip is “PFC must be peer-to-peer”: if one side pauses and the other doesn’t, frames get dropped silently, so always confirm both ends of the link and all intermediate interfaces have PFC enabled for the same CoS value.
⚠ Common exam trap
Cisco often tests the misconception that FCoE performance issues are due to VLAN or VSAN misconfiguration, when the real culprit is mismatched PFC settings between endpoints.
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
✓
Check if the PFC configuration matches on both ends and that PFC is enabled on the FCoE VLAN interfaces
D is correct because PFC (Priority Flow Control) must be consistently configured on both ends of an FCoE link to prevent frame loss. If PFC is enabled for CoS 3 on the upstream switch but not on the server's vNIC or the FCoE VLAN interfaces, the lack of lossless behavior causes intermittent timeouts and performance degradation, even if the FCoE VLAN shows no errors.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Verify that jumbo frames are enabled on the storage array
Why it's wrong here
Jumbo frames are typically required but would cause errors if missing.
- ✗
Confirm that the VSAN is appropriately sized for the number of hosts
Why it's wrong here
VSAN size rarely causes intermittent timeouts.
- ✗
Ensure that the FCoE VLAN is enabled for FCoE on the server's vNIC
Why it's wrong here
If hosts can reach storage, FCoE VLAN is already enabled.
- ✓
Check if the PFC configuration matches on both ends and that PFC is enabled on the FCoE VLAN interfaces
Why this is correct
PFC must be consistent across all hops.
Visual reference
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Same concept, more angles
3 more ways this is tested on 350-601
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which THREE factors should be considered when designing an FCoE SAN to avoid traffic loss? (Choose three.)
hard- A.Use standard Ethernet cut-through switching for all FCoE traffic.
- ✓ B.Enable priority flow control (PFC) on all FCoE-enabled interfaces.
- ✓ C.Use a dedicated FCoE VLAN that is not used for any other traffic.
- D.Disable the FIP snooping feature to reduce latency.
- ✓ E.Ensure that the FCoE Maximum Transmission Unit (MTU) is set to 2500 bytes.
Why B: Priority Flow Control (PFC) is essential in an FCoE SAN because it provides a lossless Ethernet fabric by pausing traffic on a per-priority basis. Without PFC, standard Ethernet congestion mechanisms would drop FCoE frames, which violates the lossless requirement of Fibre Channel traffic. Enabling PFC on all FCoE-enabled interfaces ensures that buffer credits are managed correctly and no frames are discarded due to congestion.
Variation 2. A network engineer is deploying FCoE in a top-of-rack design. Which statement about the required lossless Ethernet fabric is correct?
medium- A.Data Center Bridging Exchange (DCBX) automatically enables PFC when FCoE is configured
- B.FCoE uses VLAN 1002 by default for all traffic
- C.FCoE frames are limited to 1500 bytes to avoid fragmentation
- ✓ D.Priority Flow Control (PFC) must be enabled to create lossless CoS
Why D: FCoE requires a lossless Ethernet fabric to prevent frame drops, which would corrupt Fibre Channel traffic. Priority Flow Control (PFC), defined in IEEE 802.1Qbb, provides per-priority pause mechanisms to create lossless CoS (Class of Service) queues, ensuring no frames are lost due to congestion. Without PFC, standard Ethernet's best-effort delivery would cause frame drops, breaking FCoE's reliability.
Variation 3. Refer to the exhibit. An engineer is troubleshooting poor FCoE performance. The exhibit shows output from the FCoE interface. Which observation indicates a potential issue?
hard- ✓ A.PFC frames received is high compared to PFC frames sent
- B.The CRC error count is zero
- C.Data frames are 1500 bytes, which is too small for FCoE
- D.The admin port mode is F instead of NP
Why A: A high count of PFC frames received compared to PFC frames sent indicates that the local interface is receiving many pause frames from the peer, which suggests the peer is experiencing congestion and is forcing the local interface to pause its transmission. This asymmetry is a classic symptom of a buffer congestion or credit starvation issue in the FCoE SAN fabric, leading to poor FCoE performance.
Last reviewed: Jun 24, 2026
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