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GS920 Series Gigabit Ehternet Switch Installation and User's Guide
Non-root Switch
Examples of
Selecting a Root
Switch
If a switch is not selected as the Loop Prevention root switch, then it is a
non-root switch. A non-root switch updates the hop count in the Loop
Prevention frames before forwarding them. It also maintains a timer for
each of the switch's ports.
When receiving a Loop Prevention frame at a port, the switch sets the port
timer to 16 seconds. The timer decreases every second. When another
Loop Prevention frame is received at the port, the switch refreshes the
port timer again to 16 seconds. If all of the switch's port timers reach zero,
then the switch interprets the situation as a topology change where the
root switch no longer is present. The non-root switch changes itself back
to a root switch and begins to broadcast Loop Prevention frames which
starts a new selection process for a new root switch.
Root Switch Based on Highest Priority
In the example shown in Figure 63, Switch A is elected as the root switch
because this switch has the highest priority.
x
Figure 63. Case 1: Selecting a Root Switch
Root Switch Based on Highest Priority/Lowest MAC Address Value
In the example shown in Figure 64, Switch A has a higher priority than
Switch C. Switch A and Switch B have the same switch priority. Since
Switch A's MAC address is smaller than Switch B's, Switch A is elected as
the root switch.
Figure 64. Case 2: Selecting a Root Switch
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