"CCNA Exam Question: Frame Check Sequence Failure"

"Frame Check Sequence Failure"


A frame that enters a switch fails the Frame Check Sequence.

Which two interface counters are incremented? (Choose two.)



Click on the arrows to vote for the correct answer

A. B. C. D. E.


Whenever the physical transmission has problems, the receiving device might receive a frame whose bits have changed values.

These frames do not pass the error detection logic as implemented in the FCS field in the Ethernet trailer.

The receiving device discards the frame and counts it as some kind of input error.

Cisco switches list this error as a CRC error.

Cyclic redundancy check (CRC) is a term related to how the FCS math detects an error.

The input errors includes runts, giants, no buffer, CRC, frame, overrun, and ignored counts.

The output below show the interface counters with the show interface s0/0/0 command:

Routerwshow interface 0/0/0
Seriale/@/@ is up, line protocol is up
Hardware is maT
Description: Link to R2
Internet address is 1¢.1.1.1/30
MTU 1500 bytes, BH 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
s-output onitted-.
5 minute output rate @ bits/sec, @ packets/sec
268 packets input, 24889 bytes, @ no buffer
Received @ broadcasts, @ runts, @ giants, @ throttles
© input errors, @ CRC, @ frame, @ overrun, © ignored, @ abort
251 packets output, 23498 bytes, @ underruns
@ output errors, @ collisions, @ interface resets
@ output buffer failures, @ output buffers swapped out
@ carrier transitions  OCDsup OSReup DTReup RTSsup CTS=up

When a frame fails the Frame Check Sequence (FCS), it indicates that the data in the frame was corrupted during transmission. This usually happens due to noise or interference on the transmission medium, such as a cable or wireless signal.

When a switch receives a corrupted frame, it will typically increment two interface counters: input errors and CRC (Cyclic Redundancy Check).

Input errors indicate the number of times that an interface has received a frame that failed the FCS check, while CRC errors specifically indicate the number of frames that failed the FCS check.

Therefore, the correct answers are A. input errors and D. CRC.

Giants, runts, and frames are also types of errors that can occur in network transmissions. Giants are frames that exceed the maximum allowable frame size, while runts are frames that are smaller than the minimum allowable frame size. However, these errors are not typically associated with FCS failures.

In summary, when a frame fails the FCS check, the switch will increment the input errors and CRC counters on the receiving interface.