One last thing we need to cover before we can move on-the actual forwarding techniques used by switches. LAN switching forwards (or filters) frames based on their hardware destination- the MAC address. There are three methods by which frames can be forwarded or filtered. Each method has its advantages and disadvantages, and by understanding the different LAN switch methods available, you can make smart switching decisions.
Here are the three switching modes:
Store-and-forward With the store-and-forward mode, the complete data frame is received on the switch's buffer, a cyclic redundancy check (CRC) is run, and then the destination address is looked up in the MAC filter table.
Cut-through With the cut-through mode, the switch waits for only the destination hardware address to be received and then looks up the destination address in the MAC filter table.
FragmentFree FragmentFree is the default mode for the Catalyst 1900 switch; it is sometimes referred to as modified cut-through. The switch checks the first 64 bytes of a frame for fragmentation (because of possible collisions) before forwarding the frame.
Figure 4.7 shows the different points where the switching mode takes place in the frame. The different switching modes are discussed in detail next.
Figure 4.7: Different switching modes within a frame
With the store-and-forward switching method, the LAN switch copies the entire frame onto its onboard buffers and computes the CRC. Because it copies the entire frame, latency through the switch varies with frame length.
The frame is discarded if it contains a CRC error, if it's too short (fewer than 64 bytes including the CRC), or if it's too long (more than 1518 bytes including the CRC). 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 to its destination.
This is the mode used by modern Catalyst switches, and it further allows for quality of service to be applied to the frame by reading additional data. QoS is covered in detail in Chapter 8.
With the cut-through switching method, the LAN switch copies only the destination address (the first six bytes following the preamble) 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 cut-through switch provides reduced latency because it begins to forward the frame as soon as it reads the destination address and determines the outgoing interface.
Some switches can be configured 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 is a modified form of cut-through switching. In FragmentFree mode, the switch waits for the collision window (64 bytes) to pass before forwarding. If a packet has an error, it almost always occurs within the first 64 bytes. FragmentFree mode provides better error checking than the cut-through mode, with practically no increase in latency.