Understanding Priority Disconnect
Installing and Managing Power over Ethernet (PoE)
page 3-26
OmniSwitch 7700/7800 Hardware Users Guide
April 2005
Priority Disconnect is Enabled; Same Priority Level on All PD Ports
Reminder.
Priority disconnect examples are only applicable when there is inadequate power remaining to
power an incoming device.
When a PD is being connected to a port with the
same priority level
as all other ports in the slot, the physi-
cal port number is used to determine whether the incoming PD will be granted or denied power. Lower-
numbered ports receive higher priority than higher-numbered ports. In other words, a PD connected to
Port 1 will have a higher power priority than a PD connected to Port 2; a PD connected to Port 23 will
have a higher power priority than a PD connected to Port 24. In order to avoid a power budget deficit,
another port in the slot is disconnected. In determining which port to power off, the system software
disconnects the port with the highest physical port number.
In the example below, there are only 2 watts available in the current PoE power budget. When the addi-
tional 3.5W powered device is added to the configuration, the system software must determine whether the
device will be granted or denied power. Because the incoming device is being connected to a lower-
numbered physical port, it receives higher priority over the existing PDs and is granted power. Mean-
while, to avoid a deficit in the power budget, the PD connected to the highest physical port number (in this
case, Port 18) is disconnected.
Priority Disconnect Example 1: Feature is Enabled; Same Priority Level on All PD Ports
1x
3x
5x
7x
9x
11
x
13x
15x
17x
19x
21x
23x
OK1
OK2
24x
2x
!
Low
Low
Low
Low
Low
Low
Low
Port 18
Port 2
Existing Powered
Devices (PDs)
Current power budget
has 2 watts available
for incoming PDs
Incoming Powered
Device (PD) requiring
approx. 3.5 watts
UPS
UPS
Power Shelf
