Cut-through switching
A device that operates in cut-through switching mode starts forwarding a frame even before the whole frame has been received. The amount of time the device takes to start forwarding the packet (referred to as the switch's latency) is on the order of a few microseconds only, regardless of the packet size. Cut-through latency provides the latency details.
Default Behavior on Devices that Support Cut-through Switching
| Supported Device | Default Mode |
|---|---|
| ICX 7750 | Cut-through switching mode |
| ICX 7850 | Store-and-forward mode |
On devices that support cut-through switching, the default switching method for packet
forwarding can be changed using the
[no]store-and-forward command. In the store-and-forward mode, the data packets are not forwarded until
the device receives the whole frame and checks its integrity. However, there are many
factors to consider when selecting which switching method is best for your environment.
In some cases it is desirable to change from the default method and configure a device
to store-and-forward. You must save the configuration and reload for the change to
take effect.
The following table describes some of the differences in how packets are handled depending on the switching method.
- If there is any over-subscription on the egress port, either due to speed mismatch or network topology, the device will buffer the packets, and the forwarding behavior will be similar to store-and-forward mode.
- If an FCS error is determined when the packet is processed by the ingress pipe, it is dropped at the end of the ingress pipe. When an FCS error is determined after the packet transmission to the egress port has begun, it is transmitted with a faulty CRC. When an FCS error is determined during a packet transmission, the packet is truncated.
- Forwarding from fast-speed ports to slower ports is equivalent to store-and-forward (has to be stored first). Forwarding from slower speed ports to faster ports is also equivalent to store-and-forward (to avoid underrun).
- Cut-through switching is not enabled on 1G ports.
- ICX 7850-48C devices do not support cut-through switching on either 1G or 10G ports. Furthermore, when cut-through is enabled, while 1G and 10G ports remain in store-and-forward mode (the default), flow control is disabled on these ports in both directions.
- Cut-through minimum packet size is 128 bytes.
- Features that are based on the packet length are not supported since the packet is transmitted before being fully received.