CCNA Practical Studies (Cisco Certification & Training)

As mentioned earlier, switches use the same technique as transparent bridges to learn of hosts on each of their ports. They, too, forward frames only out the port where the destination MAC address resides. Because of the large number of ports that switches have, several switching technologies have been developed to speed the process of switching frames from the source port to the destination port.

Three types of switching technologies exist:

• Cut-through The switch performs the MAC address table query as soon as the destination address field in the frame header is received. The first part of the frame can be sent out the destination port before the remainder of the incoming frame is received. The major concern with this technique is that the switch can forward incomplete or damaged frames, consequently introducing more unneeded traffic and bandwidth utilization. This is the most commonly used type of switching in the industry.

• Store-and-forward The switch completely receives the whole frame before forwarding the frame. This enables the switch to check the frame check sequence (FCS) before forwarding the frame. This form of switching enables the switch to drop frames that are incomplete or damaged, thus reducing unneeded traffic. Bridges use store-and-forward technology.

• FragmentFree This technique is similar to cut-through switching, in that it forwards part of the frame before receiving the entire frame, but the switch waits for 64 bytes to be received before forwarding the first part of the frame. This is to check for collisions, which should be detected in the first 64 bytes of the frame. FragmentFree switching checks for collisions, but it does not check the FCS in the frame. On some switches, including the Catalyst 1900 series, FragmentFree is also known as FastForward.