Spanning Tree
UM RedundConfig RSPL
Release 2.0 02/2013
3.4 The Rapid Spanning Tree
Protocol
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3.4.3 Spanning Tree Priority Vector
To assign roles to the ports, the RSTP bridges exchange configuration
information with each other. This information is known as the Spanning Tree
Priority Vector. It is part of the RSTP BPDUs and contains the following
information:
Bridge identification of the root bridge
Root path costs of the sending bridge
Bridge identification of the sending bridge
Port identifiers of the ports through which the message was sent
Port identifiers of the ports through which the message was received
Based on this information, the bridges participating in RSTP are able to
determine port roles themselves and define the port states of their own ports.
3.4.4 Fast reconfiguration
Why can RSTP react faster than STP to an interruption of the root path?
Introduction of edge-ports:
During a reconfiguration, RSTP switches an edge port into the
transmission mode after three seconds (default setting) and then waits for
the “Hello Time” to elapse, to be sure that no bridge sending BPDUs is
connected.
When the user ensures that a terminal device is connected at this port and
will remain connected, there are no waiting times at this port in the case
of a reconfiguration.
Introduction of alternate ports:
As the port roles are already distributed in normal operation, a bridge can
immediately switch from the root port to the alternate port after the
connection to the root bridge is lost.
Communication with neighboring bridges (point-to-point connections):
Decentralized, direct communication between neighboring bridges
enables reaction without wait periods to status changes in the spanning
tree topology.