An access point keeps rebooting or shows a low power warning. How do you work through a PoE problem?
Show the answer
Say this first
Check what the port is delivering and what the switch has left. show power inline shows the module budget,
each port's operational state, the power drawn, the detected class and the configured maximum. Common causes:
the switch power budget is exhausted, the port is capped below what the AP needs, the AP negotiated a lower
class and disabled radios, or the cable or a passive patch in the path is bad.
Go deeper
Look at the switch
Output
SW1# show power inlineModule Available Used Remaining (Watts) (Watts) (Watts)------ --------- -------- ---------1 437.0 431.8 5.2Interface Admin Oper Power Device Class Max (Watts)--------- ------ ---------- ------- ------------------- ----- ----Gi1/0/1 auto on 6.3 IP Phone 8845 2 30.0Gi1/0/2 auto on 25.5 AP-2F-EAST 4 30.0Gi1/0/3 auto off 0.0 n/a n/a 30.0
Only 5.2 W remain. A new PoE+ AP plugged into Gi1/0/3 cannot be powered.
Fixes, in order
Add or upgrade the power supply, or move devices to a switch with budget.
Set priorities so critical devices win when the budget runs short (platform feature, often
power inline priority or a stack power policy).
Make sure the port is not capped:Cisco IOS
interface GigabitEthernet1/0/3 power inline auto max 30000
The value is in milliwatts.
Check the AP side: many APs drop to a reduced mode (fewer spatial streams, USB off, one radio off) when
they get PoE instead of PoE+.
Inspect the cable. Long or poor runs and old injector setups cause voltage drop and flapping.
Planning rule
Sum the class maximum of every device you expect, not the typical draw, unless you use CDP or LLDP
negotiation and have measured. Leave headroom for a power supply failure if the switch has two.
They will ask next
What does power inline static do differently from power inline auto?
Static pre-allocates the power to that port even before a device is connected, guaranteeing it. Auto
allocates on demand after detection and classification, first come first served.
How do you stop a port from ever supplying power?
power inline never on the interface. Useful on ports facing other switches or untrusted equipment.
Traps
Looking only at the port and not the module budget. The port can be fine and the switch empty.