Store-and-Forward

Store-and-Forward
With the store-and-forward switching method, the LAN switch copies the entire frame onto its
onboard buffers and computes the CRC. Because it copies the entire frame, latency through the
switch varies with frame length.
The frame is discarded if it contains a CRC error, if it’s too short (fewer than 64 bytes
including the CRC), or if it’s too long (more than 1518 bytes including the CRC). If the frame
doesn’t contain any errors, the LAN switch looks up the destination hardware address in its
forwarding or switching table and determines the outgoing interface. It then forwards the
frame to its destination.
This is the mode used by modern Catalyst switches, and it further allows for
quality of service (QoS) to be applied to the frame by reading additional data.
QoS is covered in detail in Chapter 20, “Quality of Service (QoS).”