Installing and Managing Power over Ethernet (PoE)
Understanding Priority Disconnect
OmniSwitch 7700/7800 Hardware Users Guide
April 2005
page 3-27
Priority Disconnect is Enabled; Incoming PD Port has Highest Priority Level
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 a
higher priority level
than all other ports in the slot, the
incoming PD will automatically be granted power over the other devices, regardless of its physical port
number.
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 first selects the port with the lowest configured priority level. For
example, if a
critical priority
device is being added to a slot in which five existing devices are attached to
high priority
ports and one device is attached to a
low priority
port, the low priority port is automatically
disconnected, regardless of its physical port number.
If all existing devices are attached to ports with the same lower priority level, the system software discon-
nects the port with both the lowest priority level
and
the highest physical port number. For example, if a
critical priority device is being added to a slot in which six existing devices are attached to high priority
ports, the high priority port with the highest physical port number is automatically disconnected.
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
critical
priority port, and all existing devices are connected to
high
priority ports, one of the high priority ports
must disconnect in order to accommodate the incoming critical device. According to the priority discon-
nect rules outlined above, the system software selects the port with the lowest priority level
and
the high-
est physical port number. In this example, Port 12 is disconnected.
Priority Disconnect Example 2: Feature is Enabled; Incoming PD Port has Highest Priority Level
1x
3x
5x
7x
9x
11
x
13x
15x
17x
19x
21x
23x
OK1
OK2
24x
2x
!
Critical
High
High
High
High
High
High
Port 24
Port 1
Existing Powered
Devices (PDs)
Port 12
Incoming Powered
Device (PD) requiring
approx. 3.5 watts
Current power budget
has 2 watts available
for incoming PDs
UPS
UPS
Power Shelf
