Ethernet - role of FCS field

The Frame Check Sequence or FCS found in the trailer of an Ethernet frame is used to ensure that a received frame has not been corrupted during transit. Communication at Layer 2, which is where Ethernet resides, is performed on a hop-by-hop basis. The FCS is actually recalculated at every hop. If the FCS fails when transmitted from one router to another, the frame is simply dropped. It is up to higher-layer protocols to recover from such a failure.

Contrast this to similar mechanisms such as the checksum field in a TCP header which ensures end to end data integrity. Unlike in the case of the FCS, in the event of a checksum failure, TCP will request the data to be resent.