A Brief of LAN Switching

LAN switches forward Ethernet frames-they just have to decide when to forward them and when not to. Most switch logic relates somehow to the source and destination MAC addresses inside the Ethernet frame headers of the frames sent through the LAN.Switch logic is also dependent on the type of MAC address used. So, a brief review of Ethernet address can help shed some light on how LAN switches work.

The EEE defines three general categories of MAC address on Ethernet:

• Unicast address: – A MAC address that identifies a single LAN interface card. Today, most cards use the MAC address that is burned into the card.
• Broadcast address: – The most often used IEEE group MAC address, the broadcast address, has a value of FFFF.FFFF.FFFF (in hexadecimal notation). The broadcast address implies that all devices on the LAN should receive and process a frame sent to the broadcast address.
• Multicast address:- Frames sent to unicast address are destined for a single device; frames sent to a broadcast address are sent to all devices on the LAN. Frames sent to multicast addresses are meant for all devices that care to receive the frame, meaning that all devices might receive the frame, none, or some number in between. Some applications need to communicate with multiple other devices. By sending one frame, all the devices that care about receiving the data sent by that application can process the data, and the rest can ignore it.

With these reminders of the three types of Ethernet MAC addresses, you can appreciate the logic used by a LAN switch. A switch listens for frames that enter all its interfaces. After receiving a frame, about a switch decides whether to forward a frame and, if so, out which port(s). switches basically perform three tasks:

• Learning: - The switch learns MAC addresses by examining the source MAC address of each frame the bridge receives. By learning, the switch can make good forwarding choices in the future.
• Forwarding or filtering:- The switch decides when to forward a frame or when to filter (not forward) it based on the destination MAC address. The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.
• Loop prevention:- The switch creates a loop-free environment with other bridges by using Spanning Tree Protocol (STP). Having physically redundant links helps LAN availability, and STP prevents the switch logic from letting frames loop around the network indefinitely, congesting the LAN.