Switch Counters Incremented When a Frame Fails FCS
Short answer
AD A frame failing the Frame Check Sequence increments the CRC counter and the aggregate input errors counter on the receiving switch interface.
Question
Key Takeaways
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.
Common mistake: 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.
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 standard show 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.
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?
Why are runts and giants not counted for a bad FCS frame?
Discussion 20 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 20
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DatBroNZ 👍 37input errors: total of many counters, including all below frame: Frames on illegal format. Can be caused by collisions giants: Frames that exceeded the maximum size (1518 bytes) CRC: Received frames that did not pass the FCS math runts: Frames that did not meet the minimum size (64 bytes). Can be caused by collisions Therefore CRC and INPUT ERRORS will be increased.
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EdgingTheCCNA 👍 4When a frame enters a switch and fails the Frame Check Sequence (FCS), it means that the integrity of the frame has been compromised, usually due to data corruption during transmission. The FCS is a checksum value included in the frame to detect errors. If the calculated FCS does not match the one in the frame, the switch will discard the frame to prevent forwarding potentially corrupted data. A CRC (Cyclic Redundancy Check) error occurs when the FCS does not match the calculated value, indicating data corruption in the frame. The CRC counter specifically tracks these types of errors, which happen when the FCS mismatch occurs.
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ZUMY 👍 3Switch port status Runts Gaint Input errors CRC Output errors Frame
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divine_love 👍 2Explanation When a frame fails the Frame Check Sequence (FCS), it indicates that there has been a data transmission error. This typically increments the input errors counter and the Cyclic Redundancy Check (CRC) counter on the interface of the switch where the frame was received. The input errors counter is incremented for any errors at the input, and the CRC counter specifically tracks errors detected by the FCS.
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anoop4487 👍 2Input Errors – This counter tracks errors in incoming frames, including CRC errors and other physical layer issues. CRC – A CRC (Cyclic Redundancy Check) error indicates that the frame has failed the FCS validation.
Show 15 more comments
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aosroyal 👍 2correct
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SyLu4N 👍 1A. Input Errors = É incrementando por ser um frame recebido e não originado do equipamento. D. CRC = O motivo pelo que o frame foi recusado.
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stevenseagal100 👍 1After reviewing it would make sense for Input Errors and CRC would be the likely indicators of the problem
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duongccna 👍 1a frame that enters a sw fails the FCS, -> FRAME, CRC
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EkaRulez 👍 1Input errors and CRC will be increased
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jat1 👍 1A AND D IS CORRECT ANSWER
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SrikanthGaddam 👍 1Input errors and CRC
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aljofer 👍 1 Answered DCorrect answer is D
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jonathan126 👍 1A and D is correct
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aljofer 👍 1 Answered DThe answer is D
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NetStef 👍 1A,D is correct.
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diegoherreras 👍 1opciones a y d
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kilticespi 👍 1It is correct
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JORGED 👍 1A,D options
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otonielsantos 👍 1a and d
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