|  | 9.7 | Switching Methods
| 9.7.2 |
Cut-through and store-and-forward LAN switching
The two main types of switching methods are store-and-forward and cut-through as shown in Figure . Depending upon the switch, this may be a configurable parameter on each individual port. Store-and-forward Store-and-forward is typically the default method on most switches. Before a switch forwards an Ethernet frame out another interface (by looking up the destination MAC address in the switch’s Source Address Table) it copies the entire frame into its buffers, checks the FCS (Frame Check Sequence) against its own calculations. If the FCS and its own calculations match, the frame is forwarded out the proper port. If the FCS does not match its own calculations, then the frame is dropped. Checking the FCS takes time, i.e. causes additional latency in the switch, but all errors are filtered. Cut-through There are two types of cut-through switching available on most Cisco switches: Fast-forward Fragment-free Fast-forward switching begins to forward a frame out the proper switch port immediately after reading the layer 2 destination address and looking that address up that address in the switch’s Source Address Table. The frame will begin to be forwarded out that interface before the rest of the frame is copied into the switch. The FCS is not checked with fast-forward switching therefore there is no error checking. Fragment-free switching performs like cut-through, but waits until the first 64 bytes of the frame has been received before forwarding the first bytes of the frame out of the outgoing switch port. According to Ethernet and 802.3 specifications, collisions should be detected during the first 64 bytes of a frame. Just like fast-forward switching, in fragment-free switching the FCS is not checked, so again there is no error checking. Fragment-free switching is faster (less latency) than store-and-forward but slower (more latency) than fast-forward.