The STP prevents a loop in the local area networks. A loop can happen if you connect two or more switches with multiple connections, as shown in the following diagram:
Let's see how a loop is created:
- Station A sends a broadcast to the network. A broadcast can be an ARP, NetBIOS, or any other packet with all fs in the destination MAC address.
- Since broadcasts are forwarded to all ports of the switch, SW1 receives the broadcast from port 1 and forwards it to ports 2 and 3.
- SW2 and SW3 will forward the packets to their other ports, which will get them to ports 2 and 3 of SW4