Installing and Managing Power over Ethernet (PoE)
Understanding Priority Disconnect
OmniSwitch 7700/7800 Hardware Users Guide
April 2005
page 3-25
Understanding Priority Disconnect
The priority disconnect function differs from the port priority function described on
in that it
applies only to
the addition of powered devices (PDs) in tight power budget conditions
. Priority discon-
nect is used by the system software in determining whether an incoming PD will be granted or denied
power when there are too few watts remaining in the PoE power budget for an additional device. For
example, if there are only 2 watts available in the current PoE power budget and a user plugs a 3.5W
powered device into a PoE port, the system software must determine whether the device will be powered
on. Based on priority disconnect rules, in some cases one or more existing devices may be powered down
in order to accommodate the incoming device. In other cases, the incoming device will be denied power.
Priority disconnect rules involve the port priority status of an incoming device (i.e., low, high, and criti-
cal), as well as the port’s physical port number (i.e., 1–24). Understanding priority disconnect rules is
especially helpful in avoiding power budget deficits and the unintentional shutdown of mission-critical
devices when PDs are being added in tight power budget conditions. For detailed information on how
priority disconnect uses port priority and port number criteria for determining the power status of incom-
ing PDs, refer to the illustrated examples on pages
through
.
Reminder.
Priority disconnect only applies when there is inadequate power remaining in the power
budget for an incoming device.
For information setting the priority disconnect status, refer to the section below. For information on setting
the port priority status (a separate function from priority disconnect), refer to
.
Setting Priority Disconnect Status
By default, priority disconnect is
enabled
in the switch’s system software. For information on changing
the priority disconnect status, refer to the information below.
Disabling Priority Disconnect
When priority disconnect is disabled and there is inadequate power in the budget for an additional device,
power will be denied to
any
incoming PD, regardless of its port priority status (i.e., low, high, and criti-
cal) or physical port number (i.e., 1–24).
To disable priority disconnect, use the
command. Because the switch
allows you to set priority disconnect status on a slot-by-slot basis, be sure to specify the slot number in the
command line. For example, the syntax
lanpower 2 priority-disconnect disable
disables the priority disconnect function on the OS7-ENI-P24 module installed in slot 2.
Enabling Priority Disconnect
To enable priority disconnect, use the
command. Be sure to specify the slot
number in the command line. For example, the syntax
lanpower 9 priority-disconnect enable
enables priority disconnect on the OS7-ENI-P24 module installed in slot 9.
