STP Configuration
Chapter 21. STP Configuration
21.1 STP Overview
21.1.1 Function of STP
Spanning Tree Protocol (STP) is applied in loop network to block some undesirable
redundant paths with certain algorithms and prune the network into a loop-free tree, thereby
avoiding the proliferation and infinite cycling of the packet in the loop network.
21.1.2 Protocol Packets of STP
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 for calculating spanning trees and maintaining the spanning
tree topology.
Topology change notification (TCN) BPDUs, used for notifying concerned devices of
network topology changes, if any.
21.1.3 Basic concepts in STP
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
at a certain interval and other devices just forward the BPDUs. This mechanism ensures
topological stability.
Root Port
On a non-root bridge device, the root port is the port nearest to the root bridge. The root
port is responsible for communication with the root bridge. A non-root-bridge device has one
and only one root port. The root bridge has no root port.
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