FCS
acronymStands for: Frame Check Sequence
The four-octet checksum at the end of an Ethernet frame, a CRC-32 the receiver recomputes to detect corruption.
A frame that fails it is dropped without a word; the only trace is an FCS or CRC error counter on the receiving port.
The check covers the frame from the destination address to the end of the padding and leaves out the preamble, the start delimiter and itself: in one vendor's words, the MAC 'computes the CRC32 over the frame bytes that include the source address, destination address, length, data, and pad'. Because a switch that adds or removes a VLAN tag changes those bytes, it recomputes the FCS as the frame leaves.
Ethernet does not retransmit a failed frame; whatever runs above it, usually TCP, notices the loss and recovers, slowly. So a rising FCS counter is a physical-layer symptom read at Layer 2: a damaged cable, a dirty fibre connector, a failing optic, electrical noise. Cisco's counter documentation adds the classic non-physical cause, noting that FCS errors 'can also indicate a duplex mismatch', and on the half-duplex side of a mismatched link they climb with load.
Also known as: frame check sequence, ethernet crc, crc-32, fcs errors, crc errors