6 Cut-Through

Cut-through is defined in the IEEE 802.1Qcc standard and is the ability to start forwarding a frame before the complete frame has been received.

This contrasts with the normal Store-and-Forward mode where the complete frame is received and checked for errors before the forward decision is taken.

Cut-through is configured per egress port, per queue and is disabled by default, which means that all the frames are forwarded in the Store-and-Forward mode.

Both unicast and multicast traffic is supported in Cut-through mode.

Cut-through forwarding is only possible if the following conditions are met:

  • In case of multicast traffic, the cut-through decision is taken based on whether all the egress ports for a particular frame are enabled for cut-through.
  • The speed of the egress port must be equal to or less than the speed of the ingress port(s).
  • The egress port(s) must be idling, that is, not currently transmitting anything.

If one or more of these conditions are not met, the frame is forwarded in Store-and Forward mode.

Cut-through can forward frames with errors, because the Frame Check Sequence (FCS) cannot be calculated before the entire frame has been received.