Switching Loops (Spanning Tree Protocol)

Spanning Tree Protocol 

Switching Loops

A Layer-2 switch belongs to only one broadcast domain, and will forward both broadcasts and multicast out every port but the originating port.
When a switching loop is introduced into the network, a destructive broadcast storm will develop within seconds. A storm occurs when broadcasts are endlessly forwarded through the loop. Eventually, the storm will choke off all other network traffic.
Consider the following example: 

If HostA sends out a broadcast, SwitchD will forward the broadcast out all ports in the same VLAN, including the trunk ports connecting to SwitchB and SwitchE. In turn, those two switches will forward that broadcast out all ports, including the trunks to the neighboring SwitchA and SwitchC. 

The broadcast will loop around the switches infinitely. In fact, there will be two separate broadcast storms cycling in opposite directions through the switching loop. Only powering off the switches or physically removing the loop will stop the storm.