Rapid Spanning Tree
Redundancy L2E
Release
4.1
03/08
4.6
The Rapid Spanning Tree Protocol
61
4.6 The Rapid Spanning Tree
Protocol
The RSTP takes over the calculation of the tree structure by the STP un-
changed. RSTP merely changes parameters, and adds new parameters and
mechanism that speed up the reconfiguration in the case of a failure.
The ports play a significant role in this context.
4.6.1 Port roles
RSTP assigns each bridge port one of the following roles (see fig. 34):
X Root port
This is the port at which a bridge receives data packets with the lowest
path costs from the root bridge.
If there are multiple ports with the same low path costs, the bridge
identifier determines which port is the root port.
If there are multiple ports with the same low path costs and the same
bridge identifier, the port identifier determines which port is the root port
(see fig. 30).
The root bridge does not have a root port.
X Designated port
The bridge in a network segment that has the lowest root path costs is the
designated bridge. If multiple bridges have the same root path costs, then
the bridge with the smallest value for the bridge identifier becomes the
designated bridge. The port on this bridge that connects it to a network
segment that leads from the root bridge, is the designated port.
X Edge port
Every network segment in which there are no additional RSTP bridges is
connected with exactly one designated port. This designated port is then
also an edge port. The distinction of an edge port is the fact that it does
not receive any RST BPDUs (Rapid Spanning Tree Bridge Protocol Data
Unit).
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