Switch Counters Incremented When a Frame Fails FCS
A frame that enters a switch fails the Frame Check Sequence. Which two interface counters are incremented? (Choose two.)
Community Insight
The exam tests whether you know that CRC is the specific FCS-failure counter while input errors is the rollup counter that includes CRC — the trap is picking one counter instead of the pair.
When a received frame on a switch interface fails the Frame Check Sequence, the interface increments both the CRC error counter and the aggregate input errors counter. This page explains why CRC and input errors rise together on a FCS failure, and why runts, giants, and frame counters describe different problems.
Learners most often pick D alone (CRC) or add E (runts), but a runt is a sub-64-byte frame problem and a giant is an oversized frame problem; neither is implied by a mere FCS checksum mismatch, and input errors must also be counted.
Community Discussion (22 comments)
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Expert Analysis
Why the Answer Is Correct
A frame that fails the FCS has been received with corrupted content, so the switch increments the CRC counter, which Cisco defines as received frames that did not pass the FCS math. Because CRC errors are tallied inside the aggregate input errors rollup, the input errors counter increments as well, making A and D the required pair. As DatBroNZ summarized, input errors is "total of many counters, including all below," and CRC is frames "that did not pass the FCS math." This is the standardshow interfaces behavior on Catalyst switches for a checksum failure on ingress.Why the Other Options Are Wrong
The frame counter tracks frames received with an illegal format, often caused by collisions or cabling defects, not a checksum mismatch. Giants counts frames larger than the 1518-byte maximum, while runts counts frames smaller than the 64-byte minimum — a FCS failure can happen to a perfectly sized frame, so neither size-based counter is guaranteed to move. Choosing only CRC (as aljofer did) is incomplete because the input errors rollup is also incremented by the same event.Community Comment Notes
Consensus across the comments favors AD: EdgingTheCCNA explains that a failed FCS means "the integrity of the frame has been compromised" and the switch discards it, while anoop4487 notes input errors "tracks errors in incoming frames, including CRC errors." divine_love likewise states the failure "increments the input errors counter and the Cyclic Redundancy Check (CRC) counter." Only aljofer selected D alone, and SyLu4N's Portuguese note makes the same AD distinction between the rollup counter and the specific cause.Official Reference
Exam Strategy
In show interfaces output, remember that input errors is an umbrella: it sums CRC, frame, overrun, and ignore errors, so any specific ingress error almost always pulls input errors up with it. When a two-counter question names both a rollup and a specific cause, pick both.
Frequently Asked Questions
Why does input errors increase along with CRC on an FCS failure?
Input errors is an aggregate counter that includes CRC, frame, overrun, and ignore errors, so any CRC error automatically raises it too.
Why are runts and giants not counted for a bad FCS frame?
Runts and giants count frames that violate the 64/1518-byte size limits; an FCS failure can occur on a frame of normal size.
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