In practical Local Area Networks, it is common that switches are interconnected with multiple links for redundancy. When switches are interconnected, the network will not fail completely, even if one of the connected links fails in-between switches. 

When switches are interconnected for redundancy as shown below, a single frame copy is forwarded multiple times throughout the redundant links, and the serious network problem can occur, which is known as ethernet loop.

Now, what is STP and how does STP prevent the Ethernet loops 

Spanning Tree Protocol (STP) is a Layer 2 protocol that runs by default on Cisco switches. IEEE 802.1D defines the features of STP. When you have redundant paths in network, loops should be avoided and this is the main function of STP. Loops are not at all acceptable in a network.

A Spanning Tree is an inverted tree. The Root switch is a special bridge at the top of the Spanning Tree (inverted tree). The Ethernet connections are then branched out from the root switch, connecting to other switches in the Local Area Network (LAN).

The complete STP operation done by three easy steps:

a) Selecting the Root Switch 

b) Selecting the Root Port 

c) Selecting the Designated and Blocking port.

In order to provide path redundancy and avoid a loop condition, STP defines a tree that spans all the switches in an Ethernet network. STP forces some redundant data paths into a standby (blocked) state and leaves other paths in a forwarding state. If a link in the forwarding (DP) state becomes unavailable, STP reconfigures the network and reroutes data paths through the activation of the appropriate standby path.