What an Ethernet Switch Does ?

1. They copy Ethernet frames from one switch port to another, based on the Media Access Control (MAC) addresses in the Ethernet frames.
2. Switches make traffic forwarding decisions based on the 48-bit media access control (MAC) addresses used in LAN standards, including Ethernet.
3. As each frame is received on each port, the switching software looks at the source address of the frame and adds that source address to a table of addresses that the switch maintains. This is how the switch automatically discovers which stations are reachable on which ports.
4. When the switch receives a frame that is destined for a station address that it hasn’t yet seen, the switch will send the frame out all of the ports other than the port on which is arrived. This process is called flooding,
5. The switch will not forward a frame destined for a station that is in the forwarding database onto a port unless that port is connected to the target destination. In other words, traffic destined for a device on a given port will only be sent to that port; no other ports will see the traffic intended for that device.
6. Switches automatically age out entries in their forwarding database after a period of time—typically five minutes—if they do not see any frames from a station. Therefore,if a station doesn’t send traffic for a designated period, then the switch will delete the forwarding entry for that station. This keeps the forwarding database from growing full of stale entries that might not reflect reality.
7. A packet sent to the broadcast address (the address of all 1s) is received by every station on the LAN. Since broadcast packets must be received by all stations on the network, the switch will achieve that goal by flooding broadcast packets out all ports except the port that it was received on, since there’s no need to send the packet back to the originating device. This way, a broadcast packet sent by any station will reach all other stations on the LAN.
8. On a network with switches connected together to form a packet forwarding loop, packets will circulate endlessly around the loop, building up to very high levels of traffic and causing an overload. The looped packets will circulate at the maximum rate of the network links, until the traffic rate gets so high that the network is saturated. Broadcast and multicast frames, as well as unicast frames to unknown destinations, are normally flooded to all ports in a basic switch, and all of this traffic will circulate in such a loop. Once a loop is formed, this failure mode can happen very rapidly, causing the network to be fully occupied with sending broadcast, multicast, and unknown frames, and it becomes very difficult for stations to send actual traffic.