Ruijie RG-S2900G-E Series, Configuration Manual

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Configuration Guide MSTP Configuration
Figure-4
If all of these three switches enable the Spanning Tree protocol, they will select
switch A as the root bridge by exchanging BPDU message. Once Switch B
detects that two ports are connected to Switch A, it will select the port with the
highest priority as the root port, while another one is selected as the alternate
port. Meanwhile, Switch C detects that it can reach Switch A through Switch B
or directly. However, Switch C discovers that the cost of the path from Switch B
to Switch A is lower than that directly (For the costs corresponding to various
paths, refer to table ***), so Switch C selects the port connected with Switch B
as the root port, while the one that connected with Switch A as the alternate port.
Various ports enter the corresponding status after their roles are determined. As
a result, the network topology is generated as shown in Figure 5.
Figure-5
If the active path between Switch A and Switch B fails, the backup path will work.
Consequently, the network topology is generated as shown in Figure 6.