Huawei Quidway S7700 Configuration Manual - Ethernet page 399
Smart routing switch
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Quidway S7700 Smart Routing Switch
Configuration Guide - Ethernet
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Issue 01 (2011-07-15)
A 16-bit PID is composed of a 4-bit port priority and a 12-bit port number.
The PID is used when the designated port needs to be selected. That is, when the root
path costs and the sender BIDs of two ports are the same, the port with a smaller PID
is selected as the designated port. As shown in
BIDs of port A and port B on S2 are the same. Port A has a smaller PID, and is thus
selected as the designated port on the local segment. The port priority that can be
configured on a Huawei device ranges from 0 to 240, with the step 16. That is, the port
priority can be 0, 16, or 32. By default, the port priority is 128.
Path cost
A path cost is port-specific, which is used by STP/RSTP as a reference to select a link.
STP/RSTP calculates the path cost to select the robust link and blocks redundant links to
trim the network into a loop-free tree topology.
On an STP/RSTP-capable network, the accumulative cost of the path from a certain port
to the root bridge is the sum of the costs of the segment paths into which the path is separated
by the ports on the transit bridges.
Port roles
– STP-capable port
– Root port
The root port is the port that is nearest to the root bridge. The root port is determined
based on the path cost. Among all the ports where STP is enabled on the network
bridge, the port with the smallest root path cost is the root port. There is only one
root port on an STP-capable device, but there is no root port on the root bridge.
– Designated Port
The designated port on a switching device forwards bridge protocol data units
(BPDUs) to the downstream switching device. All ports on the root bridge are
designated ports. A designated port is selected on each network segment. The device
where the designated port resides is called the designated bridge on the network
segment.
– RSTP-capable port
Compared with STP, RSTP has two additional types of ports, namely, the alternate port
and backup port. More port roles are defined to simplify the knowledge and deployment
of STP.
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
8 STP/RSTP Configuration
Figure
8-1, the root path costs and sender
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