Spanning-Tree is a layer 2 loop prevention protocol. When switches are in a redundant network, there are chances of loops that can affect the network. To prevent this STP blocks some redundant links in the topology. Itelects a root bridge with the help of BPDUs that switches send every 2 secs. Root bridge act as a reference point and all the ports of the root bridge act as a Designated port. On the Non Root Switches, Ports can either be Designated Port, Root port or Alternate/Blocked port.