Switched Forwarding
Switches route data based on the destination MAC address contained in the frame’s header. This approach
allows switches to replace Layer 2 devices such as hubs and bridges.
After a frame is received and the MAC address is read, the switch forwards data based on the switching mode
the switch is using. This strategy tends to create very low latency times and very high forwarding rates.
Switches use three switching modes to forward information through the switching fabric:
Store−and−forward• Cut−through• FragmentFree•
Tip Switching fabric is the route data takes to get from the input port on the switch to the output port
on the switch. The data may pass through wires, processors, buffers, ASICs, and many other
components.
Store−and−Forward Switching
Pulls the entire packet received into its onboard buffers, reads the entire packet, and calculates its cyclic
redundancy check (CRC). It then determines if the packet is good or bad. If the CRC calculated on the packet
matches the CRC calculated by the switch, the destination address is read and the packet is forwarded out the
correct port on the switch. If the CRC does not match the packet, the packet is discarded. Because this type of
switching waits for the entire packet before forwarding, latency times can become quite high, which can result
in some delay of network traffic.
Cut−Through Switching
Sometimes referred to as realtime switching or FastForward switching, cut−through switching was developed
to reduce the latency involved in processing frames as they arrive at the switch and are forwarded on to the
destination port. The switch begins by pulling the frame header into its network interface card buffer. As soon
as the destination MAC address is known (usually within the first 13 bytes), the switch forwards the frame out
the correct port.
This type of switching reduces latency inside the switch; however, if the frame is corrupt because of a late
collision or wire interference, the switch will still forward the bad frame. The destination receives the bad
frame, checks its CRC, and discards it, forcing the source to resend the frame. This process will certainly
waste bandwidth; and if it occurs too often, major impacts can occur on the network.
In addition, cut−through switching is limited by its inability to bridge different media speeds. In particular,
some network protocols (including NetWare 4.1 and some Internet Protocol [IP] networks) use windowing
technology, in which multiple frames may be sent without a response. In this situation, the latency across a
switch is much less noticeable, so the on−the−fly switch loses its main competitive edge. In addition, the lack
of error checking poses a problem for large networks. That said, there is still a place for the fast cut−through
switch for smaller parts of large networks.
FragmentFree Switching
Also known as runtless switching, FragmentFree switching was developed to solve the late−collision problem.
These switches perform a modified version of cut−through switching. Because most corruption in a packet
occurs within the first 64 bytes, the switch looks at the entire first 64 bytes to get the destination MAC
address, instead of just reading the first 13 bytes. The minimum valid size for an Ethernet frame is 64 bytes.
By verifying the first 64 bytes of the frame, the switch then determines if the frame is good or if a collision
occurred during transit.
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