Planet Networking & Communication WGS3-2820 User Manual page 135

In this example, only the default STP values are used.
B
Port 2
The switch with the lowest Bridge ID (switch C) was elected the root bridge, and the ports were selected to give a high port cost
between switches B and C. The two (optional) Gigabit ports (default port cost = 4) on switch A are connected to one (optional)
Gigabit port on both switch B and C. The redundant link between switch B and C is deliberately chosen as a 100 Mbps Fast
Ethernet link (default port cost = 19). Gigabit ports could be used, but the port cost should be increased from the default to
ensure that the link between switch B and switch C is the blocked link.
A
Port 1
Designated Port
Root Port
Port 1
Port 3
LAN 2
After Applying the STA Rules
User's Manual of WGS3-2820/WGS3-5220
LAN 1
Port 3
Root Bridge
Port 2
Designated Port
Root Port
C
Blocked
Port 2
135
Port 1
Designated Bridge
Port 3
LAN 3