Cisco® CCNA Exam Cram Notes : Switching methods

IV. LAN Switching Technologies

2. Switching methods

The Catalyst 1900 and 2820 series switches support three types of switching methods:

1. FastForward (Cut-through): In this type of switching, the packet is forwarded as soon as the destination address is read. This has least latency.

2. FragmentFree ( Modified cut-through): This type of switching is useful when your network is experiencing large number of collisions. FragmentFree switching has a latency in between FastFoward and the Store-and -Forward.

3. Store-and-Forward: This method stores the entire frame and checks for errors before forwarding it on to another port. Store-and-forward has the highest latency compared with both FastForward and FragmentFree.

The default switching method used by Catalyst 1900 series switches is FastForward.

Store-and-Forward switching: Here the LAN switch copies the entire frame into its buffers and computes the CRC. The frame is discarded if there are any CRC errors. Giant ( more than 1518 bytes) and Runt (less than 64 bytes) frames are also dropped, if found

Cut-through switching: In cut-through switching switch copies into its memory only the destination MAC address (first 6 bytes of the frame) of the frame before making a switching decision. A switch operating in cut-through switching mode reduces delay because the switch starts to forward the Ethernet frame as soon as it reads the destination MAC address and determines the outgoing switch port. Problem related with cut-through switching is that the switch may forward bad frames.

Fragment-free (runtless switching): switching is an advanced form of cut-through switching. The switches operating in cut-through switching read only up to the destination MAC address field in the Ethernet frame before making a switching decision. The switches operating in fragment-free switching read at least 64 bytes of the Ethernet frame before switching it to avoid forwarding Ethernet runt frames (Ethernet frames smaller than 64 bytes).

Errdisable is a feature that automatically disables a port on a Catalyst switch. When a port is error disabled, it is effectively shut down and no traffic is sent or received on that port. You can have automatic recovery with errdisable recovery command, such as:

"errdisable recovery cause security-violation" in global configuration mode.

Alternatively, you can manually recover from the errdisable state by entering the “shutdown” and “no shut down” interface configuration commands. When error disable recovery is enabled, the interface automatically recovers from the err-disabled state, and the device retries bringing the interface up.

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