HP V1910 Switch Series User Manual page 55

 Select IRF from the navigation tree of Switch C to enter the page of the Setup
Figure 29 Configure a stack port on Switch C
In the Port Settings area, select the check box before GigabitEthernet1/0/1.

Click Enable.

Now, Switch C becomes a member switch.
# On Switch D, configure local port GigabitEthernet 1/0/1 connecting with Switch B as a stack port.
 Select IRF from the navigation tree of Switch D to enter the page of the Setup tab.
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tabFigure 29.