CRC or FCS errors indicate what type of issue on a network link?

• A. Layer-3 routing failures.
• B. Application layer data corruption.
• C. Data link layer control frame misalignment.
• D. Layer-1 frame integrity errors often caused by bad cabling or signaling.

Answer: (D)

CRC or FCS checks are used to verify that a frame received on the link hasn’t been corrupted. When a frame fails the CRC, it means the bits got altered in transit, so the frame is considered invalid. The root cause of those corrupt frames is almost always a physical-layer problem—bad cabling or signaling issues such as damaged cables, poor connectors, electrical interference, or faulty transceivers. That’s why this points to a Layer-1 frame integrity issue.

In practice, you’d start by checking the physical link: replace the cable, reseat connectors, test with a known-good transceiver, and look for sources of interference or mismatched cable types. The other options describe higher-layer problems (routing, application data) or framing concerns that don’t inherently explain a CRC failure as clearly as a physical-layer fault.