How The Spanning Tree Protocol Works; Cbpdus At Work - 3Com corebuilder 3500 Implementation Manual

How the Spanning
Tree Protocol Works

CBPDUs at Work

Using the Spanning Tree Protocol (STP), bridges transmit messages to
each other that allow them to calculate the Spanning Tree topology.
These messages are special packets called Configuration Bridge Protocol
Data Units (CBPDUs), or configuration messages.
CBPDUs do not propagate through the bridge as regular data packets do.
Instead, each bridge acts as an end station, receiving and interpreting
CBPDUs.
Bridge Hierarchy
The CBPDUs help bridges establish a hierarchy (or a calling order) among
themselves for the purposes of creating a loopless network.
Based on the information in the CBPDUs, the bridges elect a root bridge,
which is at the top level of the hierarchy. The bridges then choose the
best path on which to transmit information to the root bridge.
The bridges that are chosen as the best path, called designated bridges,
form the second level of the hierarchy:
A designated bridge relays network transmissions to the root bridge
through its root port. Any port that transmits to the root bridge is a
root port.
The designated bridges also have designated ports — the ports that
are attached to the LANs from which the bridge is receiving
information.
Figure 20 shows the hierarchy of the STP bridges and their ports.

How the Spanning Tree Protocol Works

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