Pilz PSSnet SHL 8T MRP - Manual (Page 69)

Rapid Spanning Tree
PSSnet SHL - Redundancy Configuration
Pilz GmbH & Co. KG, Felix- Wankel Str. 2, 73760 Ostfildern
4.6 The Rapid Spanning Tree
Protocol
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4.6.3 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 RST BPDUs and contains the following
information:
X Bridge identifier of the root bridges
X Root path costs for the sending bridges
X Bridge identifier for the sending bridges
X Port identifiers of the ports through which the message was sent
X Port identifiers of the ports through which the message was received
Based on this information, the bridges participating in RSTP are able to
calculate port roles themselves and define the port states of their own ports.
4.6.4 Fast reconfiguration
Why can RSTP react faster than STP to an interruption of the root path?
X Introduction of edge ports
During a reconfiguration, RSTP switches an edge port into the trans-
mission mode after three seconds and then waits for the “Hello Time”
(see
table 12) to elapse, to be sure that no bridge sending BPDUs is
connected.
When the user is sure that a terminal device is connected at this port and
will remain connected, he can switch off RSTP at this port. Thus no
waiting times occur at this port in the case of a reconfiguration.
X 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.
X Communication with neighboring bridges (point-to-point connections)
Decentralized, direct communication between neighboring bridges
enables immediate reaction to status changes in the spanning tree
architecture.