Which field is used to detect errors in a received Ethernet frame?

• A. auto-MDIX
• B. CEF
• C. Frame Check Sequence
• D. minimum frame size

Answer: (C)

Error detection in Ethernet frames is accomplished by the Frame Check Sequence, a 32-bit CRC trailer appended to the end of the frame. The sender computes a CRC over the frame’s header and payload and attaches it as the FCS. When a receiver gets the frame, it recalculates the CRC on the received data and compares it to the FCS value. If they match, the frame is considered error-free; if they don’t, the frame is discarded. This is how the network quickly detects corrupted data caused by transmission errors.

The other options don’t perform error detection in the frame itself: auto-MDIX adjusts for cable type and polarity, not data integrity; CEF is a forwarding mechanism used by routers, not an error-detection field; and the minimum frame size relates to collision detection in older Ethernet Ethernet networks, not to detecting errors in received frames.