The Rapid Spanning Tree Protocol; Port Roles - Hirschmann RS20 User Manual

Spanning Tree
6.6 The Rapid Spanning Tree
Protocol
The RSTP uses the same algorithm for determining the tree structure as
STP. RSTP merely changes parameters, and adds new parameters and
mechanisms that speed up the reconfiguration if a link or bridge becomes
inoperable.
The ports play a significant role in this context.
6.6.1

Port roles

RSTP assigns each bridge port one of the following roles
 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 equally low path costs, the bridge ID of the
bridge that leads to the root (designated bridge) decides which of its ports
is given the role of the root port by the bridge further away from the root.
If a bridge has multiple ports with equally low path costs to the same
bridge, the bridge uses the port ID of the bridge leading to the root
(designated bridge) to decide which port it selects locally as the root port
(see figure 46).
The root bridge itself does not have a root port.

Designated port:
The bridge in a network segment that has the lowest root path costs is the
designated bridge.
If more than 1 bridge has the same root path costs, the bridge with the
smallest value bridge identifier becomes the designated bridge. The
designated port on this bridge is the port that connects a network segment
leading away from the root bridge. If a bridge is connected to a network
segment with more than one port (via a hub, for example), the bridge
gives the role of the designated port to the port with the better port ID.
102
6.6 The Rapid Spanning Tree
(see figure
UM RedundConfig L2P
Release 8.0 05/2013
Protocol
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