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How LAN Switches Work


LAN switches rely on packet-switching. The switch establishes a connection between two segments just long enough to send the current packet. Incoming packets (part of an Ethernet frame) are saved to a temporary memory area (buffer); the MAC address contained in the frame's header is read and then compared to a list of addresses maintained in the switch's lookup table. In an Ethernet-based LAN, an Ethernet frame contains a normal packet as the payload of the frame, with a special header that includes the MAC address information for the source and destination of the packet.

Packet-based switches use one of three methods for routing traffic:

  • Cut-through
  • Store-and-forward
  • Fragment-free

Cut-through switches read the MAC address as soon as a packet is detected by the switch. After storing the 6 bytes that make up the address information, they immediately begin sending the packet to the destination node, even as the rest of the packet is coming into the switch.

A switch using store-and-forward will save the entire packet to the buffer and check it for CRC errors or other problems before sending. If the packet has an error, it is discarded. Otherwise, the switch looks up the MAC address and sends the packet on to the destination node. Many switches combine the two methods, using cut-through until a certain error level is reached and then changing over to store-and-forward. Very few switches are strictly cut-through, since this provides no error correction.

A less common method is fragment-free. It works like cut-through except that it stores the first 64 bytes of the packet before sending it on. The reason for this is that most errors, and all collisions, occur during the initial 64 bytes of a packet.

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