Figure 6-10 Diagram of port roles
S2 S3
A
AB
A a
S2 S3
A
AB
A
a
B
b
Alternate port
Backup port
root port
designated port
S1
root bridge
S1
root bridge
As shown in Figure 6-10, RSTP defines four port roles: root port, designated port, alternate
port, and backup port.
The functions of the root port and designated port are the same as those defined in STP.
The alternate port and backup port are described as follows:
– From the perspective of configuration BPDU transmission:
– An alternate port is blocked after learning a configuration BPDU sent by another
bridge.
– A backup port is blocked after learning a configuration BPDU sent by itself.
– From the perspective of user traffic:
– An alternate port acts as a backup of the root port and provides an alternate path
from the designated bridge to the root bridge.
– A backup port acts as a backup of the designated port and provides a backup path
from the root bridge to the related network segment.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
6 STP/RSTP Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
181
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