Cisco Network LAN Switching Technologies

The latency or rate at which packets switch through a certain switch depends on the type of Switching mode chosen.

We will be looking at the three switching modes in this blog post.
Store and forward: In the store and forward mode, the complete data frame is received on the switch’s and then  initiate a Buffer, which helps a CRC to run, making then the destination address seen  in the switch MAC filter table.




Cut-through: in this mode,  the switch will have to  waits for the destination hardware Address to be received before looking for the destination address in the
MAC filter table.
Fragment Free
The cisco Catalyst 1900 switch, is sometimes referred to as modified cut-through. This because it checks the first 64 bytes of a frame for every fragmentation before forwarding the frame to any port.
Store and Forward
Store-and-forward switching is one of three primary types of LAN switching.
With the store-and-forward switching method, the LAN switch memories
the entire frame onto its onboard buffers and computes the cyclic redundancy
check (CRC). Because it has memoried the entire frame, latency through the
switch varies with frame length.
The frame will be automatically  discarded if it contains a CRC error, if it’s too short  or if it’s too long.
If the frame doesn’t contain any errors, the LAN switch looks
up the destination hardware address in its forwarding or switching table and
determines the outgoing interface. It then forwards the frame toward its destination.
This is the mode used by the Cisco Catalyst 5000 series switches and cannot be modified on the switch.
Cut-Through (Real Time)
Cut-through switching technology  is the another important type of LAN switching technology to be employed. In this technology, the LAN switch memories only the destination address onto its onboard buffers. It then looks up the hardware destination address in the MAC switching table, determines the
outgoing interface, and forwards the frame toward its destination. A cutthrough
switch provides reduced latency because it starts the frame forwarding as soon as it lays holds on  the destination address and determines the outgoing
interface
.
In our networks we can configure Some switches to perform cut-through switching on a per-port basis until a user-defined error threshold is reached. At that point,
they automatically change over to store-and-forward mode so they will stop
forwarding the errors. When the error rate on the port falls below the threshold,
the port automatically changes back to cut-through mode.
FragmentFree (Modified Cut-Through)
FragmentFree is also known as modified form of cut-through switching technology,  here, FragmentFree  switching technology, the switch waits for the collision window to pass before forwarding frames, and If it determines packet error, it will know that it  occurs within the first 64 bytes.
FragmentFree mode provides better error checking than the cut-through
mode with practically no increase in latency. This is the default switching method for the Cisco1900 series switches.



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