Switching – Store and forward, Cut-through and Fragment free
Switches examine the source and destination MAC in a frame to build their MAC table and make their forwarding decision. Exactly how they do that is the topic of this section. You need to be aware of three switching modes: Store and Forward, Cut Through, and Fragment Free.
Store and forward
Store-and-forward switching means that the LAN switch copies each complete frame into the switch memory buffers and computes a cyclic redundancy check (CRC) for errors. CRC is an error-checking method that uses a mathematical formula, based on the number of bits (1′s) in the frame, to determine whether the received frame is errored. If a CRC error is found, the frame is discarded. If the frame is error free, the switch forwards the frame out the appropriate interface port
An Ethernet frame is discarded if it is smaller than 64 bytes in length, a runt, or if the frame is larger than 1518 bytes in length, a giant.
With cut-through switching, the LAN switch copies into its memory only the destination MAC address, which is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame on to its destination through the designated switch port. A cut-through switch reduces delay because the switch begins to forward the frame as soon as it reads the destination MAC address and determines the outgoing switch port,
In packet switched networks such as Ethernet, pure cut-through switching can only be used where the speed of the outgoing interface is less than or equal to the incoming interface speed.
The disadvantage is clearly that bad frames will be switched along with the good. Because the CRC/FCS is not being checked, we might be propagating bad frames. This would be a bad thing in a busy network, so some vendor’s support a mechanism in which the CRCs are still checked but no action is taken until the count of bad CRCs reaches a threshold that causes the switch to change to Store-and-Forward mode.
Fragment-free switching is also known as runtless switching and is a hybrid of cut-through and store-and-forward switching.
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.
This is only useful if there is a chance of a collision on the source port – so a fully switched network may not benefit from fragment free in comparison to low latency cut through switching. Frames are forwarded before any checksums can be calculated.
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