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21
Three Switching Modes
Cut-Through
In this mode the switch starts forwarding the packet to the output port as soon as
the destination address or the source-route of the incoming packet has been
resolved. This technique ensures very low latency, typically in the range of 30-100
µs. However, if errors occur on the input port during the reception of a packet, the
error will still be forwarded to the output port. Note that cut-through can only be
used in transmissions between ports which operate at 16 Mbps.
This mode is only supported by the Token-Ring ports.
Store and Forward
In this mode, the switch receives the total packet from the input port, checks it for
any errors and then starts forwarding the packet to the destination port. This
technique will ensure that no faulty packets are transmitted by output port. The
negative impact however, is higher latency, typically in the range of 40–2,000 µs
depending on the packet size. Though slower than cut-through mode, this is still
much faster that conventional bridges.
The Fast Ethernet ports support this mode only.
Auto (Adaptive Cut-Through)
This is a technique whereby the switch will automatically swap between store-and-
forward and cut-through modes based on an error threshold. If the number of
received faulty packets is low, then cut-through mode is used; if the number of
faulty packets is high, the store and forward mode is used. This provides optimized
performance but introduces variable latency.
CrossFire 8730 Switch Reference Guide, DOC-7047 v. 1.1
Switch Overview
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