Store-and-forward switching

Store-and-forward switching is one of the methods a network switch can use to forward Ethernet frames. The switch reads the entire frame into its buffer, recalculates the frame check sequence (a cyclic redundancy check carried at the end of the frame) and compares the result with the value the sender wrote. A frame that fails the check, or is too short or too long to be valid, is discarded. Only a frame that passes is sent out of the port that leads to its destination MAC address.

Its counterpart is cut-through switching, where the switch starts forwarding as soon as it has read the destination address near the start of the frame. Cut-through gives lower and more constant latency, but it can pass damaged frames on to the next device. Store-and-forward adds a delay that grows with the length of the frame, in return for stopping errors at the switch. A middle variant, often called fragment-free, reads the first 64 bytes before forwarding, since that is where collision damage commonly shows.

Exam relevance: questions in this area tend to ask which switching method checks frames for errors, or which one keeps latency lowest. Candidates are expected to link store-and-forward with error checking and higher latency, and cut-through with speed and no check of the full frame.