Frequency Required. Power Consumption Operational. Output Voltage. Power Provided. Compliant Standards. Environmental Parameters. Min Operating Temperature. Max Operating Temperature. Humidity Range Operating. After classification, the switch applies 48 to 52 VDC to the line, which is the operational voltage of telephones and wireless access points. The powered device is now fully powered and should be operational or registering with the call manager if the powered device is an IP phone.
Most earlier prestandard switches such as the Catalyst can not provide full power to newer devices such as IP phones with a color display. Bringing the phone up in reduced power mode at least allows it to operate, even though possibly with reduced features or reduced display brightness. The switch uses maintain power signature MPS to detect the continued presence of a powered device after detection and after power is applied. As long as a power signature is present, the switch provides PoE.
The powered device must maintain its power signature to maintain a PoE feed from the switch. PoE is typically removed within milliseconds after the switch no longer detects a power signature. All disconnect methods occur at the physical layer. The switch port removes power if the DC current in the current loop to the powered device is below a specific threshold for to milliseconds.
When a powered-up device is disconnected, the current between the switch port receive and send pairs drops to 0. As long as DC current is flowing between minimum and maximum thresholds, the switch determines that a powered device is connected, powered, and should continue receiving power, as shown in Figure 2.
An unreliable connection at some point between the switch and powered device can cause a false disconnect. A device that is locally powered by an AC power module is detected by the PoE switch when connected to a port.
However, the switch either provides or removes power based on a low current threshold for example, the powered device is not using power from the switch. When powered devices are locally powered no PoE , it might be appropriate in some cases to use the power inline never interface configuration command:. Some early Catalyst and switches apply a Hz signal between the receive and send pairs of the Ethernet port. A low-pass filter in the powered device loads attenuates the Hz signal level below a specific threshold.
The low-pass filter works only at Hz and does not loop back normal traffic or Ethernet control signals. The switch does not detect the attenuated Hz signal when the powered device has the low-pass filter between the receive and send pairs. This means that a powered device is still connected and powered on, and that power to the device should continue, as shown in Figure 3. When the powered device is disconnected, theHz signal loading is removed from the line and the Hz signal goes up beyond a specific threshold.
The switch detects the signal level increase and recognizes that a powered device has been disconnected. The switch removes PoE from the Ethernet port. Power to a device can continue when an MPS signal is present, even if the Ethernet link is down. The disconnect method depends on the connected powered device. A Cisco switch reports when a powered device is connected or disconnected and when an Ethernet link state changes.
This example shows a Cisco IP phone that was disconnected from the Ethernet port and then reconnected. Note Unreliable plugs or jacks at any point between the phone and switch can cause the same disconnections.
The actual power requirement can be advertised by the powered device, and the unused class power is returned to the switch power budget. CDP is typically enabled by default. When a Cisco powered device phone or WAP powers on, CDP sends a message that tells the switch how much power the powered device actually needs.
The switch CPU adjusts the power allocation for the port and adjusts the power budget. Without CDP, if a powered device is connected but the class is 0 default or cannot be identified, the switch must reserve the maximum per-port PoE for the powered device. This can quickly deplete the power budget and, in a worst case, could result in a total artificial power budget depletion.
Then the switch might not allocate power to a detected powered device even though power is still available. This wastes 6. An improper classification signature in the powered device can cause the same problem. This might prevent all intended devices from receiving power. Catalyst , , and newer switches monitor the actual per-port power used after powered device detection is complete and PoE is applied.
If the required power is less than the default If the requested power exceeds the power budget available for the switch, power is either be denied, or the port remains in low-power mode typically 7 W. Use the show power inline privileged EXEC command to review these factors:. To see connected Cisco devices, use the show cdp neighbor or show cdp neighbor detail privileged EXEC commands:. Note The maximum required power for this phone is milliwatts This example shows the allocated and power budget for two switches in a Stackwise stack.
Switch 1 Module 1 is a E, and switch 2 is a G. Note This is a Class-3 phone, but uses only The G is using In this example, some of the phones are early prestandard units. The indicator of standard or nonstandard varies depending the switch model and Cisco IOS version. Failing to identify a specific class is typically a sign of a prestandard powered device.
The show power inline command offers port-specific information:. It is possible that some of the prestandard non-Cisco powered devices might not be detected by a Cisco switch, but later IEEE-compliant powered devices should not have this problem when connected to a IEEE-compliant Cisco switch. CDP identifies Cisco powered devices and establishes the appropriate power budget, but non-Cisco devices do not have the advantage of CDP. If a non-Cisco powered device can be detected but the power class cannot be identified, the switch must default to class 0 and allocate the maximum power for the port.
This can cause a premature depletion of the power budget. For example, unidentified devices might need only 6 W, but they are allocated maximum PoE power up to This results in a symptom where a known-good powered device is connected to a known-good PoE port on a switch, but the powered device does not power on. Some of the newer switches, such as the Catalyst E and E can adjust the power budget according to actual measured power usage.
Some non-Cisco devices might have an excessive surge in current when first connected to a PoE port. The switch initially provides power to the port, then quickly removes power due to a momentary, overcurrent condition. The powered device appears to power on, but then quickly powers down. This error message from a Catalyst is possibly caused by an overcurrent condition:. Any current over mA is usually considered an overcurrent condition.
All PoE switches have electronic voltage and current regulators that detect an overcurrent threshold and disconnect DC power from the line to prevent damage. Cisco PoE switches have two levels of overcurrent protection:.
This prevents excessive current from being delivered by the PoE port, which could possibly result in damage to port-level components.
If the line current on a particular port increases to approximately 9 W, the switch removes power to prevent circuit damage due to overheating. Newer switches such as the Catalyst E, E, and G support up to Some earlier switches might not be able to provide The power budget is usually related to the switch power-supply capability and the amount of delivered power.
All newer switches can power a mix of 7. The Catalyst E and E switches support Switches that provide Normally the automatic electronic current regulation is completely effective, and fuse action is very seldom necessary or observed.
Fuses can either be self-resetting replacement not required or a basic, fast-acting fuse that requires replacement after burning open. Fuses are included in the PoE power supply as a safeguard, but the port-level electronic regulators are the primary method of overcurrent protection.
The self-resetting fuses are PTC positive temperature coefficient resistors. These components are a polymer-based temperature-dependent resistor, not actually a fuse. When activated by heat, PTC units increase in resistance.
PTC fuses are also sensitive to high ambient temperature inside the Ethernet switch chassis , so these devices might appear to activate more quickly when exposed to higher temperatures. The actual current threshold is a factor of both heat from circuit current and heat from ambient temperature inside the chassis. The advantage of a self-resetting fuse is, if activated, power is restored to the line when the overcurrent condition is corrected. The disadvantage of these devices is after the first time they are activated, they usually exhibit a slightly higher resistance thereafter, even when the device temperature returns to normal or ambient.
The standard, fast-acting fuses used in some switch power supplies open if a circuit in the switch fails and causes excessive current. This is unusual, but if it occurs, replace the power supply if it is replaceable or replace the switch. In normal operation, the electronic regulators limit the per-port PoE current to safe levels.
The fuses are a second level of safety. If an overcurrent condition occurs on one port, it does not affect other ports. Some Cisco switches allow manual adjustments to the power budget and per-port control of the maximum PoE power delivered to a powered device.
In this example, the power inline consumption interface configuration command adjusts the switch power budget to 7 W when the actual power required by a powered device is known but cannot be determined by IEEE classification. This prevents artificial depletion of the switch power budget:. A powered device might use more power than set by the power inline consumption command, so you must carefully make adjustments to the power budget.
An accurate power budget protects the switch from an overcurrent condition. In this example, the power budget is manually adjusted to 4 W on a specific port.
When a Class 3 phone was connected, it required more power than 4 W:. In this case, there was remaining available power at system level, so power was delivered to the powered device.
The syslog message was an alert to report the powered device exceeding the configured power budget for the port. However, the powered device did not exceed the maximum power delivery capability of the switch.
When the power budget has been manually adjusted at port-level, the show power inline privileged EXEC command shows the administratively assigned power, not the actual power used by the powered device:. To see the power used by the powered device, use this command:.
The power request levels for this powered device a Cisco phone are The phone can operate at 6. Full screen brightness requires The phone in the example is operating at 6. The Catalyst E, , E, and series PoE switches allow maximum power delivery to a powered device on a per-port basis. This allows an override of the powered device classification. In this example, the inline power static interface configuration command sets an absolute limit of 5 W on a specific port.
The powered device requires more power than is allowed by the 5 W limit, and inline power is denied:. An under-powered condition can result in a permanent loopback from a Cisco-powered device, which also error disables the port.
For more information, refer to Power Over Ethernet Solutions. All the previously listed products rely on a phone-discovery algorithm before the power is supplied to a phone.
This algorithm ensures that the switch does not supply power to a device that is not able to accept in-line power. Both of these algorithms are explained in this section. Note: It is not possible to provide a detailed explanation of the phone-discovery algorithms because certain aspects of them are proprietary. This table explains the parameters available on the three platforms to enable or disable the supply of power to the ports.
Note: There is no such thing as an 'on' mode on any of these devices. This should protect customers from accidentally damaging any Ethernet network interface card NIC cards in devices that do not expect to receive power from the network.
The port starts the phone-discovery algorithm by sending a special Fast Link Pulse FLP signal to any device that might be connected to it. The port waits to see if the special FLP signal is forwarded back by a connected device. The only devices that are designed to do this are devices that expect to receive in-line power. It is capable of doing this because it has a special relay that connects its Ethernet receive pair with its Ethernet transmit pair.
This relay is closed when no power is being supplied to the phone. Once power is supplied, this relay remains in an open state. Later on it will adjust this allocation based on what the attached Cisco IP Phone tells the switch it really needs.
The instant that the switch applies power to the port, the relay inside the phone opens and power begins to flow to the Cisco IP Phone. At this point a 'wait for link' timer in the switch starts also.
The phone has five seconds to establish link integrity on its Ethernet port. If the switch does not detect link integrity on the port within five seconds, it will shut off power to the port and start the phone-discovery process all over again.
The switch has to wait at least five seconds so that the switch has enough time to detect all devices. If the switch detects a link within the five second window, it will continue to supply power to the Cisco IP Phone until it detects a link down event.
The NMP sees this and adjusts the power allocation for the port accordingly.
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