SAN Connectivity Issue: No Link Lights on HBA and Switch

Infrastructure Operations

A systems administrator notices a newly added server cannot see any of the LUNs on the SAN. The SAN switch and the local HBA do not display any link lights. Which of the following is most likely the issue?

A single-mode fiber cable is used in place of multimode.
The switchport is on the wrong virtual SAN.
The HBA driver needs to be installed on the server.
The zoning on the fiber switch is wrong. Source Reference Answer

Community Votes

67%

33%

67% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

Link lights are physical indicators; if they are off, the issue is hardware or cabling, not logical configuration like zoning or drivers.

The scenario describes a physical layer failure indicated by absent link lights. While the suggested answer points to zoning, community consensus correctly identifies that mismatched fiber types (single-mode vs. multimode) cause complete loss of signal at Layer 1.

Choosing D (Zoning) because it relates to SAN access control, ignoring that zoning issues do not prevent physical link establishment or light transmission.

Community Discussion (5 comments)

cj207800 👍 1 Selected: A

The key detail is that there are no link lights on both the SAN switch and the HBA. This indicates a physical layer problem, which is most commonly caused by cabling issues.

ompk 👍 1

The answer is A because this is a physical layer problem, and using the wrong type of fiber optic cable can result in no connectivity, which would explain why there are no link lights on both the switch and HBA.

ccoli 👍 1

The most likely issue that prevents the newly added server from seeing any of the LUNs on the SAN is that a single-mode fiber cable is used in place of multimode. A single-mode fiber cable is a type of optical fiber cable that has a small core diameter and allows only one mode of light to propagate through it. A single-mode fiber cable can transmit data over long distances at high speeds, but it requires more expensive transceivers and connectors than multimode fiber cables. A multimode fiber cable is a type of optical fiber cable that has a larger core diameter and allows multiple modes of light to propagate through it. A multimode fiber cable can transmit data over short distances at lower speeds than single-mode fiber cables, but it is more compatible and cost-effective than single-mode fiber cables. If a single-mode fiber cable is used in place of multimode, it can cause signal loss, attenuation, or mismatch between the devices.

surfuganda 👍 1 Selected: D

D. The zoning on the fiber switch is wrong. In a Storage Area Network (SAN) environment, zoning is a method of partitioning or segmenting the SAN fabric to control access to specific devices or resources such as LUNs. If the newly added server cannot see any of the LUNs on the SAN and both the SAN switch and the local HBA do not display any link lights, it suggests that there might be an issue with the zoning configuration. Zoning needs to be properly configured to allow communication between the server's HBA and the LUNs on the SAN. If the zoning is incorrect, the server will not be able to access the LUNs, resulting in the observed behavior.

AzadOB 👍 1 Selected: D

D. The zoning on the fiber switch is wrong. If the SAN switch and the local HBA do not display any link lights, it indicates a physical connectivity problem between the server and the SAN. Zoning on the fiber switch is a common cause of such issues. Zoning determines which devices can communicate with each other over the Fibre Channel fabric. If the zoning is incorrect, the server may not be able to establish a connection with the SAN and access the LUNs. Therefore, checking and correcting the zoning configuration on the fiber switch would likely resolve the issue.

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Expert Analysis

Why the Answer Is Correct

The absence of link lights on both the Host Bus Adapter (HBA) and the SAN switch port definitively indicates a Layer 1 (Physical) connectivity failure. Option A is correct because mixing single-mode and multimode fiber results in a complete loss of optical signal due to core diameter mismatches and lensing issues, preventing the transceivers from establishing a link.

Why the Other Options Are Wrong

Option B (Virtual SAN) and Option D (Zoning) are logical configurations that operate above the physical layer; incorrect settings would allow link lights to remain on but result in no visibility of LUNs. Option C (Driver) affects the operating system's ability to recognize the HBA, but the hardware ports themselves should still negotiate a physical link with the switch, showing link lights.

Community Comment Notes

Comments [1], [2], and [3] correctly highlight that 'no link lights' signifies a physical problem, specifically citing cable type mismatches. Comments [4] and [5] incorrectly argue for zoning despite the physical symptoms, demonstrating a common confusion between logical access controls and physical connectivity diagnostics.

Official Reference

https://www.cisco.com/c/en/us/support/docs/switches/multi-layer-switch-series-catalyst-6500/20892-how-to-troubleshoot-fibre-channel.html

Exam Strategy

Always check the status lights first. If link lights are off, look for physical causes (cables, transceivers, power). If link lights are on but devices aren't communicating, look for logical causes (zoning, masking, IP configuration).

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