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Spanning tree protocol (STP) is a protocol conforming to IEEE 802.1d. It aims to 
eliminate loops on data link layer in a local area network (LAN). Devices running 
this protocol detect loops in the network by exchanging packets with one another 
and eliminate the loops detected by blocking specific ports until the network is 
pruned into one with tree topology. As a network with tree topology is loop-free, 
it prevents packets in it from being duplicated and forwarded endlessly and 
prevents device performance degradation.

Currently, in addition to the protocol conforming to IEEE 802.1d, STP also refers to 
the protocols based on IEEE 802.1d, such as RSTP, and MSTP.

STP uses bridge protocol data units (BPDUs), also known as configuration 
messages, as its protocol packets.

STP identifies the network topology by transmitting BPDUs between STP compliant 
network devices. BPDUs contain sufficient information for the network devices to 
complete the spanning tree calculation.

In STP, BPDUs come in two types:

■

Configuration BPDUs, used to calculate spanning trees and maintain the 
spanning tree topology.

■

Topology change notification (TCN) BPDUs, used to notify concerned devices of 
network topology changes, if any.

1 Root bridge

A tree network must have a root; hence the concept of "root bridge" has been 
introduced in STP.

There is one and only one root bridge in the entire network, and the root bridge 
can change alone with changes of the network topology. Therefore, the root 
bridge is not fixed.

Upon network convergence, the root bridge generates and sends out 
configuration BPDUs periodically. Other devices just forward the configuration 
BPDUs received. This mechanism ensures the topological stability.

2 Root port

On a non-root bridge device, the root port is the port with the lowest path cost to 
the root bridge. The root port is used for communicating with the root bridge. A