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9.3 STP
9.3.1 Overview
Spanning Tree helps avoid loops in the network to protect the network from broadcast storms,
and provide link redundancy backup.
STP is a network protocol based on IEEE 802.1d. It is a protocol that ensures a loop-free
topology for local area network and provides backup redundant links. The devices under this
protocol discover the loops in the network by communicating with each other, and selectively
block some ports, and eventually establish a spanning tree structure without loops, so as to
prevent the decline of the message processing capacity of the devices due to the continuous
proliferation and endless circulation of messages in the loop network.
STP protocol message
To implement spanning tree function, switches in the network transfer BPDUs (Bridge Protocol
Data Unit) between each other to exchange information. BPDUs carry the information that is
needed for switches to calculate the spanning tree.
The network topology is determined by BPDU transmission among devices. There are two
types of BPDUs of STP protocol:
−
Configuration BPDU: It is used for spanning tree calculation and spanning tree
topology maintenance.
−
TCN BPDU (Topology Change Notification BPDU): It is used to notify the changes of
the network topology structure.
Basic concepts of STP
◼
Bridge ID
The bridge ID contains both bridge priority and MAC address, in which the bridge priority is a
configurable parameter. The smaller the bridge ID, the higher the bridge priority. The root
bridge is the bridge with the smallest bridge ID.
◼
Root bridge
Root bridge acts as the root of a tree. There is only one root bridge in the network and it is
changeable according to the network topology changes.
Initially, all devices regard themselves as the root bridges. They generate their own
configuration BPDUs and send them out periodically. When the network topology becomes
stable, only the root bridge device can send configuration BPDUs out and other devices can
only forward these BPDUs.
Chapter 9 Device management 33