Hirschmann GREYHOUND GRS1020, Справочное руководство

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Redundancy
12.4 The Rapid Spanning Tree Protocol
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UM Config GRS
Release 8.0 09/2019
12.4 The Rapid Spanning Tree Protocol
The RSTP uses the same algorithm for determining the tree structure as STP. When a link or bridge
becomes inoperable, RSTP merely changes parameters, and adds new parameters and
mechanisms that speed up the reconfiguration.
The ports play a significant role in this context.
12.4.1 Port roles
RSTP assigns each bridge port one of the following roles (see figure 40) :

Root Port:
This is the port at which a bridge receives data packets with the lowest path costs from the root
bridge.
When 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.
When 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 36) .
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.
When 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. When 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.

Edge port
Every network segment with no additional RSTP bridges is connected with exactly one
designated port. In this case, this designated port is 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 Units).

Alternate port
When the connection to the root bridge is lost, this blocked port takes over the task of the root
port. The alternate port provides a backup for the connection to the root bridge.