3Com CoreBuilder 3500, Implementation Manual, Page: 131 / 592

How the Spanning Tree Protocol Works 131
Reconfiguring the
Bridged Network
Topology
STP reconfigures the bridged network topology when any of the
following events occur:
■
Bridges are added or removed.
■
The root bridge fails.
■
You change any of the bridging parameters that influence the
topology decision.
Resulting Actions
Whenever a designated bridge detects a topology change, it sends a
Topology Change Notification Bridge Protocol Data Unit (BPDU) through
its root port. This information is eventually relayed to the root bridge.
The root bridge then sets the Topology Change Flag in its CBPDU so that
the information is broadcast to all bridges. It transmits this CBPDU for a
fixed amount of time to ensure that all bridges are informed of the
topology change.
If a port changes from the blocking state to the forwarding state as a
result of the topology change, STP sends the topology information to all
the ports before that port starts forwarding data. This delay prevents
temporary data loops.
When a network reconfiguration occurs, a bridge flushes all dynamic
addresses from its address table. This action ensures that the bridge
learns the correct addresses and paths and continues to forward packets
to the correct LANs.