Low Power Mode on Cisco Switches
What is Low Power Mode?
When connecting Cisco Meraki Access Point (AP) to several Cisco Switch models that provides energy to your AP, sometimes you may run into an insufficient power scenario if your Cisco switch is configured with CDP defaults only. This scenario is known as Low Power Mode.
PoE Standards
| Industry Standard | IEEE Standard | Description |
|
PoE |
802.3af | This is the original PoE and provides up to 15.4 Watts. This now is also known as Legacy PoE. |
| PoE+ | 802.3at | This standard provides up to 30 Watts. It requires LLDP working together with it when providing above 15.4 Watts. |
| PoE++ | 802.3bt | At the moment, this is the latest standard and provides up to 100 Watts. It requires LLDP working together with it when providing above 30 Watts |
Low Power Mode examples
As a general best practice, always check your AP and Switch datasheets and make sure they are compliant to the same or compatible standards.
Scenario One:
An MR44 can operate consuming different power levels: 30W max (PoE+) or 15W max (PoE); this AP prefers to operate in PoE+ but if connected to a switch that doesn't support either CDP or LLDP, it will fall back to PoE. Note that CDP isn't mandatory in PoE+, as per IEEE standards but LLDP negotiation is generally used to negotiate power allocation above 15.4W. Therefore, a Low Power Mode scenario would occur here.
This scenario may happen because Cisco Meraki Access Points may need to use LLDP in addition to CDP while some Cisco switches don't enable LLDP by default. You must ensure that LLDP is supported and enabled on the switch and at the Ethernet port where your AP is connected.
Scenario Two:
An MR57 can operate between PoE++ and PoE+ power levels. MR57 requires extra power to perform additional features like USB support, for example that elevates power consumption above 30W.
The MR57 will prefer to operate at PoE++ level but would fall back to PoE+ if it is connected to a switch that doesn't support both CDP and LLDP. Note that once more CDP isn't mandatory in PoE++, as per IEEE standard however, LLDP must be used to negotiate power levels above 30W as per the same standard.
If an MR57 is connected to a switch that doesn't support both CDP and LLDP, it would operate in Low Power Mode.
How to enable LLDP in a Cisco Switch
To enable LLDP globally on a switch running IOS:
Switch# configure terminal Switch(config)# lldp run Switch(config)# end
Next, enable LLDP on the specific interfaces where the access points will be connected. Follow the example below for IOS devices, substituting "interface_id" with the actual designation for the interface in question:
Switch# configure terminal Switch(config)# interface interface_id Switch(config-if)# lldp transmit Switch(config-if)# lldp receive Switch(config-if)# end
To renegotiate power, either reboot the Access Point in Dashboard or shutdown and then re-enable the port on the switch:
Switch# configure terminal Switch(config)# interface interface_id Switch(config)# shutdown Switch(config)# no shutdown Switch(config)# end
Alternatively, PoE can be disabled and then re-enabled without shutting down the port itself with the "power inline" command:
Switch# configure terminal Switch(config)# interface interface_id Switch(config)# power inline never Switch(config)# power inline auto Switch(config)# end
At this point, the switch and access point should be able to negotiate up to the required power level. If this is still not occurring, make sure the switch port is allowing up to 30W of power for the connected device:
Switch# configure terminal Switch(config)# interface interface_id Switch(config-if)# power inline max 30000 Switch(config-if)# end
Note: On newer Cisco switches, substitute "power inline max 30000" for "power inline static max."
For more information, please refer to the knowledge base article on Low Power Mode.