Understanding Power over Ethernet (PoE)

Power over Ethernet (PoE) is a technology that allows switches to pass electrical power along with data on Ethernet cables. This is usually done on the four extra wires in UTP cables, the ones that are not used for data transmission.

PoE is generally used to power up non-critical network devices in order to reduce cabling necessities in the office. Such devices include:

• IP phones
• Video conferencing cameras
• Surveillance cameras
• Wireless access points

PoE is supported only on special switch models, which are usually more expensive. On a Cisco switch, PoE can be configured in Interface Configuration mode by using the power inline command (this is on by default, but it can be disabled using the no power inline command).

You can learn more about PoE in our Cisco CCNA course.

In converged internetworks, Cisco Catalyst switches interact with Cisco IP phones in the following three different ways:

1. VLAN tagging
2. Extended trust settings
3. Inline power delivery

VLAN tagging is based on the switchport voice vlan interface configuration command. Extended trust settings are based on the switchport priority extend interface configuration command.

Cisco IP phones can use an external power cube to draw their power, or they can draw their power from the switch to which they are connected. This power is sent within the Ethernet cable connecting the switch and the IP phone. The following are two methods for providing ILP:

• IEEE 802.3af-2003
• Cisco Inline Power

IEEE 802.3af-2003 and Cisco Inline Power Overview

IEEE 802.3af-2003 is a ratified version of the original IEEE 802.3af standard. This was ratified in 2003, hence the name 802.3af-2003. The IEEE 802.3af-2003 Power over Ethernet (PoE) standard defines terminology to describe a port that acts as a power source (PSE) to a powered device (PD), defines how a powered device is detected, and defines two (2) methods of PoE to the discovered powered device.

IEEE 802.3af-2003 power may be delivered using a PoE-capable Ethernet port, which is referred to as an End-Point PSE, or by a mid-span PSE that can be used to deliver PoE in the event an existing non-PoE-capable Ethernet switch is used. The mid-span PSE is described later in this section.

IEEE 802.3af-2003 is an open standard that describes five power classes to which a device can belong. The default power classification within IEEE 802.3af-2003 delivers 15.4 W per power device. The five 802.3af-2003 power classes are listed below in Table 1:

Table 1.  IEEE 802.3af-2003 Power Classes

	Min. Power Levels Output at PSE	Max. Power Levels at the PD	Usage
0	15.4 W	0.44 to 12.95 W	Default
1	4.0 W	0.44 to 3.84 W	Optional
2	7.0 W	3.84 to 6.49 W	Optional
3	15.4 W	6.49 to 12.95 W	Optional
4	N/A	N/A	Reserved

Cisco ILP is a proprietary approach. The IEEE 802.3af standard is actually based on this method of PoE, which was available before PoE was standardized. Cisco has also extended power management extensions using CDP negotiation to Cisco IEEE 802.3af-2003-compliant devices to further optimize PSE power management. Cisco Catalyst switches support both ILP and IEEE 802.3af-2003.

Discovering Powered Devices

Before providing power, the switch needs to determine whether the port is connected to a power-capable device. Cisco ILP and the IEEE 802.3af-2003 standard use different power detection methods, both of which are supported by the switch.

IEEE 802.3af-2003 uses a Direct Current (DC)-powered device detection method. The DC detection method applies a DC current and detects the presence of a PD by measuring the load applied by the PD. The switch (PSE) will expect to see a 25 kΩ (Kilo Ohm) resistance between the pairs in order for the device to be considered a valid PD. If the PSE does not detect a valid 25 kΩ resistor, power is not applied to the port.

Unlike the IEEE 802.3af-2003 standard, Cisco ILP uses Alternating Current (AC) for PD detection in conjunction with a low-pass filter that allows the phone discovery signal to loop back to the switch but prevents 10/100 or 1000 Mbps frames from passing between the receive and transmit pairs. PD discovery operates in the following manner for Cisco ILP:

1. The switch (PSE) sends a special tone, called a Fast Link Pulse (FLP), out of the port
2. The FLP goes to the PD, such as the Cisco IP phone
3. The PD connects the transmit line to the receive line using a low-pass filter
4. The FLP is looped back to the switch, indicating it is ILP-capable
5. When the switch receives the returning FLP, it applies power to the line
6. The switch port comes up within 5 seconds and the PD boots

NOTE: The FLP will only be looped back when the PD is unpowered (i.e. has not received power). This allows the switch (PSE) to know that the device requires power.

ILP device discovery is illustrated below in Figure 1:

Fig. 1.  Inline Power Device Discovery

Using either the Cisco ILP or IEEE 802.3af-2003 method, if the PD is a Cisco IP phone, it uses CDP to tell the switch (PSE) how much power it wants. The CDP message contains an ILP Type/Length/Value (TLV) field that informs the Cisco Catalyst switch (PSE) of the actual power required by the device.

If the power is less than the default 15.4 W, the PSE acknowledges the request with its available power and modifies the PSE’s power budget. If the requesting PD exceeds the power budget for the line card or switch, the port either is powered down or remains in low-power mode.

DC detection differs from AC detection in that AC detection transmits a low-frequency AC signal (a low-pass filter) and expects the same signal to be received back on the receive pair. DC detection applies a DC and detects the presence of a PD by measuring the load applied by the PD. 

Supplying Power to Power-Capable Devices

Once the powered device has been detected, the PSE needs to supply power. The IEEE 802.3af-2003 standard states that power may be delivered by an end-point PSE, using either the active data wires of an Ethernet port or the spare wires, to a PD. An end-point PSE, such as a PoE-capable switch, may implement either scheme. It should be noted that even if a device supports both methods of providing power, only one mechanism may be used to deliver power to a PD.

With the IEEE 802.3af-2003 standard, there are two modes that can be used: mode A and mode B. In mode A, pins 1 and 2 form one side of the 48 VDC and pins 3 and 6 form the other side. These are the same pairs used for data transmission. In mode B, pins 4 and 5 form one side of the DC supply and pins 7 and 8 provide the return. These are the unused pairs.

Cisco ILP is provided over the data pairs, as is the case with IEEE 802.3af-2003 mode A. The default ILP allocation is 10 W. However, once the inline device is enabled, it will use CDP to adjust its power to the actual requirement. This enables the PD and PSE to negotiate their respective capabilities in order to explicitly manage how much power is required for the device and how the PSE-capable switch manages the allocation of power to individual PDs.

Disconnecting Power

The PSE is required to detect when the PD has been disconnected in order to ensure that power is withdrawn from a port before a non-powered device, such as a workstation or laptop, is connected to the switch port.

The IEEE 802.3af-2003 standard defines two mechanisms for disconnecting power once a device has failed: DC disconnect and AC disconnect. The DC disconnect method detects when PD current falls below a given threshold (5 to 10 mA) for a given time (300 msec to 400 msec). The AC Disconnect superimposes a small AC voltage on the power and measures the resulting AC current. If the impedance is above 26.25 kΩ (Kilo Ohms), power is shut off. With Cisco ILP, the PoE ports have a power disconnect mechanism that will remove power from the port if the Ethernet link status is down.

Additional Real-World Implementation

The IEEE 802.3at-2009 PoE standard, sometimes called ‘POE+,’ provides up to 25.5 W of power, although some vendors have announced products that claim to comply with the new IEEE 802.3at-2009 standard and offer up to 50 W of power over a single cable by utilizing all four pairs in the cable.

The IEEE 802.3at-2009 standard also specifies two types of PSEs: endspans and midspans. Endspans are simply PoE-capable Ethernet switches, such as Cisco Catalyst 3750, 4500, and 6500 series switches. Midspans are power injectors that stand between a regular switch and the PD, injecting power without affecting the data. Endspans use pairs 2 and 3 (i.e pins 1, 2, 3, and 6) to send power to the PD. These are the data pairs. Midspans use pairs 1 and 4 (i.e. pins 4, 5, 7, and 8) to send power to the PD. These are the spare pairs.

While Cisco Catalyst switches support both the IEEE 802.3af-2003 and ILP PoE methods, it is important to remember the differences between these two in order to differentiate between them. These differences, which are described in the previous section, include the following:

• The amount of power that is available to the connected device
• The method used for device discovery
• The way that power is removed from the wire when a PD is removed

Configuring Power over Ethernet

Cisco PoE-capable Catalyst switches are configured to supply power on a per-interface or per-port basis using the power inline [auto [max <max-wattage>] | never | static [max <max-wattage>]] interface configuration command. By default, in PoE-capable switches, the default is auto (enabled) and the maximum wattage is 15400 milliwatts.

The [max <max-wattage>] allows the administrator to limit the power allowed on the port. The range is 4000 to 15400 milliwatts. If no value is specified, the maximum is allowed. The [never] keyword is used to disable device detection and disable power to the port.

The [static] keyword is used to enable PD detection and to pre-allocate or reserve power for a switch port before the switch discovers the PD. This is used when connecting to PDs that cannot communicate with the PSE using any of the discovery methods that are described earlier in this section. These advanced PoE configuration options are beyond the scope of the SWITCH exam requirements and will not be described or illustrated in this chapter.

Verifying Power over Ethernet

The show power inline [interface | consumption default | module switch-number] command is used to display the PoE status for the specified PoE port or for all PoE ports.

The [consumption default] option is used to display the power allocated to devices connected to PoE ports. The [module switch-number] keywords are applicable when the switches are stacked together. These keywords can be used to limit the display of ports on the specified stack member. This is beyond the scope of the SWITCH exam requirements. The following output illustrates how to verify PoE status using the show power inline command:

Catalyst-Switch-1#show power inline   Module   Available     Used     Remaining (Watts)     (Watts)    (Watts) ——   ———   ——–   ——— 1        370.0       114.9      255.1 2        370.0        34.3      335.0   Interface Admin  Oper       Power   Device              Class Max (Watts) ——— —— ———- ——- ——————- —– —- Fa1/0/1   auto   on         6.3     IP Phone 7910       n/a   15.4 Fa1/0/2   auto   on         6.3     IP Phone 7910       n/a   15.4 … [Truncated Output] …

Read the Cisco PoE notes.