3Com SuperStack II PathBuilder S330 Reference Manual page 61

Figure 42 Priority Clock Configuration Menu
b Enter the option number corresponding to the clock source you want to
designate as the priority 1 (highest priority) system reference clock to open the
Reference Clock Selection menu for that interface. Figure 43 shows the OC3
Reference Clock Selection menu. The menus for the other interfaces are similar.
Figure 43 OC3 Reference Clock Selection Menu
c If you selected a card that has multiple ports as the clock source in step b, the
Reference Clock Port Selection menu opens; proceed to step d. If you selected
a card that does not have multiple ports as the clock source in step b, skip to
step e.
d From the Reference Clock Port Selection menu, enter the option number
corresponding to the port you want to designate as the clock source.
If you are configuring a PathBuilder S310 switch, do not select unavailable ports
(ports 2-4, CBR ports 2 and 3, and OC3/STM-1) as system clocks—even though
the software lists these ports and allows you to select them.
Performing Initial System Configuration 47