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. This technique reduces latency through the switch, but decreases reliability.

The alternative to cut-through switching is store and forward.

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

Pure cut-through switching is only possible when the speed of the outgoing interface is equal to the incoming interface speed.

Use in Ethernet

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 where a store and forward switch will drop them.^[1]

The technology was developed by Kalpana the company that introduced the first Ethernet switch.^[2]

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.^[3]

Use in ATM

Cut-through routing was one of the important features of IP networks using ATM networks since the edge routers of the ATM network were able to use cell switching through the core of the network with low latency at all points. With higher speed links, this has become less of a problem since packet latency has become much smaller.

Use in InfiniBand

Cut-through switching is very popular in InfiniBand networks, since these are often deployed in environments where latency is a prime concern, such as supercomputer clusters.

References

1. ^ "Cut-Through and Store-and-Forward Ethernet Switching for Low-Latency Environments". Cisco. http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/white_paper_c11-465436.html. Retrieved 2011-11-10. 
2. ^ "Cisco to Acquire Kalpana, Leading Ethernet Switching Company". Cisco Systems, Inc.. Archived from the original on 2010-06-18. http://www.webcitation.org/5qaWOQRdn. 
3. ^ "Switches - What Are Forwarding Modes and How Do They Work?". http://support.intel.com/support/express/switches/sb/cs-014410.htm. Retrieved 2011-08-13. 

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