Specifications

Functions
108
4.4 Spanning Tree Algorithm
RS2-4R
Release 1.0 03/04
4.4.4 Rapid Spanning Tree Protocol
The exponential increase in the use of LANs also in time-critical applications
places new demands on the availability of the networks. Switch-over times in
the high seconds range for reconfiguring the network when a subcomponent
fails can no longer be tolerated. Thus it was inevitable that the once
legendary Spanning Tree Protocol had to be revised in order to keep pace
with our ever-changing world. The improvement from several seconds to less
than one second, when compared with other technical changes, represents
more an evolution or further development than a revolution.
This further development was adopted in June 2001 as IEEE 802.1w to
supplement the existing IEEE 802.1D standard and dubbed Rapid Spanning
Tree Algorithm and Protocol (RSTP).
RSTP is compatible to the traditional STP. If, however, both protocols are
used simultaneously, there are no advantages to using the faster
reconfiguration with RSTP.
RSTP does not change the tree structure calculation. RSTP only alters
parameters, adds new parameters and mechanisms that accelerate the
reconfiguration in the event of an error. The ports play a major role within this
context.
U Port roles
RSTP assigns one of the following roles to each bridge port:
D (Root-Port),
The port at which a bridge receives data packets with the lowest path
costs from the root bridge.
If there are several such ports, the bridge identification determines
which port is the root port.
If there is also more than one of these ports, the port identification
determines which port is the root port
(see Fig. 34).
The root bridge does not have a root port itself.