Cut-through switching

In computer networking, cut-through switching is a method for packet switching systems, wherein the switch starts forwarding a frame (or packet) before the whole frame has been received, normally as soon as the destination address is processed. Compared to store and forward, this technique reduces latency through the switch and relies on the destination devices for error handling. Pure cut-through switching is only possible when the speed of the outgoing interface is equal to or greater than the incoming interface speed.

Adaptive switching dynamically selects between cut-through and store and forward behaviors based on current network conditions.

Cut-through switching is closely associated with wormhole switching.

When cut-through switching is used in Ethernet, because the frame check sequence appears at the end of the frame, the switch is not able to verify the integrity of an incoming packet before forwarding it. A cut-through switch will forward corrupted packets, whereas a store and forward switch will drop them.

The technology was developed by Kalpana, the company that introduced the first Ethernet switch.

Fragment free is a variation on cut-through switching that partially addresses this problem by assuring that collision fragments are not forwarded. Fragment free will hold the frame until the first 64 bytes are read from the source to detect a collision before forwarding. This is only useful if there is a chance of a collision on the source port.

The theory here is that frames that are damaged (usually by collisions) are often shorter than the minimum valid ethernet frame size of 64 bytes. With a fragment-free buffer the first 64 bytes of each frame, updates the source MAC and port if necessary, reads the destination MAC, and forwards the frame. If the frame is less than 64 bytes, it is discarded. Frames that are smaller than 64 bytes are called runts; this is why fragment-free switching is sometimes called “runt less” switching. Because the switch only ever buffers 64 bytes of each frame, Fragment Free is a faster mode than Store and Forward, but there still exists a risk of forwarding bad frames.

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Wikipedia