STP ( Spanning Tree Protocol)

IEEE 802.1D - STP

Rapid STP - IEEE 802.1w

First to elect root bridge
  Slowest priority will be the root bridge.Bridges have same priority then smallest mac address will be the root bridge.

STP will temporally blocks the redundant paths.

PVST+ (Per Vlan Spanning Tree) - Each vlan will elect a root switch. Each vlan have its own instance.


To elect the root bridge flood the BPDU (Bridge protocol data unit) and this contains Bridge id and sender id.



After electing root bridge.


Root port - Best path to reach root birdge/switch.
Designated port - Best path to the switched segment.
Alternate port- Backup for the root port.
Backup port - Backup of the Designated port

Port states of STP .

1. Blocking state - All the ports are in this while switch initialization , will listen to the network.
2.Listening state - Will listen the traffic, port is set to forward the frame. Default time of the state is 15secs.
3.Learning state - Will listen the traffic and begin to learn the addresses from connected devices on a segment.
4.Forwarding state - Will forward the traffic as well as continue to learn the addresses from the segment.

Default priority port is - 32768