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01 The standards and their wattage
| Standard | Name | Power at PSE port | Power at PD | Pairs |
|---|---|---|---|---|
| IEEE 802.3af (2003) | PoE, Type 1 | 15.4 W | 12.95 W | 2 pairs |
| IEEE 802.3at (2009) | PoE+, Type 2 | 30 W | 25.5 W | 2 pairs |
| IEEE 802.3bt (2018) | Type 3 | 60 W | 51 W | 4 pairs |
| IEEE 802.3bt (2018) | Type 4 | 90 W | 71.3 W | 4 pairs |
Before the standard, vendors shipped proprietary inline power that detected only their own IP phones. Any switch sold as 802.3af or later performs a resistive detection (a 25 kilohm signature) before applying voltage, so plugging a non-PoE device into a PoE port is safe.
02 PD classes and negotiation
After detection the switch classifies the powered device (PD). Under 802.3af and 802.3at the PD presents a current signature mapping to class 0 through 4, and the switch reserves the class maximum: class 1 is 4 W, class 2 is 7 W, class 3 (and class 0) 15.4 W, class 4 30 W. 802.3bt adds classes 5 through 8 for 45, 60, 75 and 90 W. LLDP-MED can refine the allocation after link-up: a class 3 phone can report it needs only 6 W and return the rest to the budget.
03 Alternative A and Alternative B
Category 5 cable has four pairs; 10BASE-T and 100BASE-TX use two for data. Alternative A injects power as a common-mode voltage on the data pairs (pins 1-2 and 3-6) through the centre taps of the magnetics. Alternative B uses the spare pairs (4-5 and 7-8). A compliant PD accepts either; the PSE chooses. Gigabit links use all four pairs for data, which is why 802.3bt, needing more current, powers all four at once, and why passive injectors built for Alternative B break gigabit links.
04 Power budget math
The datasheet number that matters is the PoE power budget, the total the internal supply can deliver to ports. A 48-port PoE+ switch with a 370 W budget cannot run 48 class 4 devices (1440 W). It can run 24 at 15.4 W, or 12 at 30 W, or a mix. Switches allocate by class in port order or by configured priority and refuse the port that would exceed the budget.
Budget on class, not on measured draw. A phone drawing 5 W still reserves 15.4 W unless LLDP renegotiates it. Sum the class maximums of everything you intend to plug in, add 20 percent for growth, and pick the switch or the second power supply that covers it.
Heat follows power: a loaded PoE switch dissipates its conversion losses in the closet, so cooling and UPS runtime need checking. Many outages blamed on flaky PoE were closets that ran out of budget when more devices were added.
05 Worked example: one closet against a 740 W budget
A 48-port switch with a 740 W PoE budget serves 20 class 3 phones, 12 class 4 access points, 6 class 6 cameras and 4 class 2 badge readers. Reserving by class: 20 times 15.4 is 308 W, 12 times 30 is 360 W, 6 times 60 is 360 W, 4 times 7 is 28 W, a total of 1,056 W. The switch powers ports in order until 740 W is committed and refuses the rest; if the cameras sit on low port numbers, phones on the high ports stay dark. Enable LLDP-MED so each phone reports its real need of about 6 W, returning roughly 188 W, and set the cameras to low priority. The total falls to about 868 W, still above 740, so the second supply is needed.
| Class | Classification current | Type | Maximum at the PSE | Available at the PD |
|---|---|---|---|---|
| 2 | 16 to 21 mA | 1 | 7.0 W | 6.49 W |
| 3 | 25 to 31 mA | 1 | 15.4 W | 12.95 W |
| 4 | 35 to 45 mA | 2 | 30 W | 25.5 W |
| 6 | 36 to 44, then 9 to 12 mA | 3 | 60 W | 51 W |
switch# show power inline Module Available Used Remaining 1 740.0 W 668.0 W 72.0 W
06 Converged networks: voice, data and wireless on one switch
PoE enabled the converged network pitch of the early 2000s: retire the PBX and its separate cabling, run IP phones and wireless access points from the same switched infrastructure as the PCs, powered by the switch and backed by the same UPS. Foundry marketed a converged networks solution set around its FastIron Edge PoE switches for IP telephony and its IronPoint access points. The same argument now covers cameras, badge readers and building controls.
Convergence needs two Layer 2 features alongside power. A voice VLAN keeps phones in their own VLAN while the PC behind the phone stays in the data VLAN on the same dual-mode port. QoS marks voice packets with DSCP EF (46) and places them in a strict-priority queue on uplinks so a file transfer cannot delay them; 802.1p class of service 5 does the same job at Layer 2. Pair it with 802.1X multi-auth so phone and PC authenticate separately. Power budget, voice VLAN and authentication all land on the same access port, which is why the access and closet switching pages treat them as one design problem rather than three.
interface ethernet 1/1 inline power inline power power-by-class 3 dual-mode 20 voice-vlan 30 ! show inline power
07 Planning checklist
- Inventory devices by PoE class, not by count; note which need 802.3at or 802.3bt.
- Sum class maximums per closet; compare against the switch budget with a second power supply if offered.
- Confirm cabling is Category 5e or better and under 100 m; poor cable raises losses.
- Verify LLDP-MED is enabled so phones and access points negotiate power and voice VLAN.
- Set port PoE priority so phones and access points outlive cameras and displays in a budget squeeze.
- Size the UPS for the added load, check closet cooling, and monitor budget use over SNMP with an alert at 80 percent.
08 Power over Ethernet pitfalls and expert tips
- Passive injectors. They skip detection and put 48 V on the pairs of whatever is plugged in. Keep them out of structured cabling.
- Four-pair power on old cable. 802.3bt needs all eight conductors terminated and Category 5e or better; a run with a split pair, or a two-pair patch lead, powers at Type 1 or not at all.
- Devices that never speak LLDP. They reserve the class maximum for ever. Identify them and budget accordingly.
Poll the power Ethernet MIB (RFC 3621) so POWER-ETHERNET-MIB counters feed the same graphs as port traffic, with an alert at 80 percent of budget. On the same access port, authenticating phone and PC separately and the voice and data VLAN split are configured together with power priority, so review all three in one change rather than three tickets.
09 Questions
Can I plug a non-PoE device into a PoE port?
Yes. Standards-based switches apply power only after detecting a valid PD signature resistance, so a laptop or an old printer never receives voltage. Passive PoE injectors are not standards-based, skip detection and can damage equipment.
Why does my 48-port PoE switch stop powering devices after about 24?
The power budget is exhausted. Many 48-port models ship with roughly 370 W, enough for 24 class 3 devices at 15.4 W. Check the reserved wattage per port, enable LLDP-MED so devices reserve less, or add the second power supply.
Does PoE work over 100 m?
Standards assume up to 100 m of Category 5e or better and specify the PD figure after that loss. Beyond 100 m data fails before power does. Poor terminations and thin patch cords increase resistance and can starve high-power devices.
What is the difference between PoE, PoE+ and PoE++?
PoE (802.3af) gives 15.4 W per port, PoE+ (802.3at) 30 W, and PoE++ is a marketing label for 802.3bt Type 3 (60 W) or Type 4 (90 W). Higher types are backward compatible with lower-class devices.
Do I need a separate voice VLAN?
It is strongly recommended. A voice VLAN separates phone signalling and media from PC traffic on the same port, allows QoS marking to be trusted from phones only, and keeps the phone subnet stable while the data VLAN changes.
Does LLDP-MED reduce the power a switch reserves?
Yes, on switches that implement it. After classification the powered device sends an LLDP power TLV stating its requirement in 0.1 W steps, and the switch lowers the allocation from the class maximum to that figure. A class 3 phone reserving 15.4 W typically drops to 6 or 7 W.