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| 3.4 |  | Identifying Physical Layer Problems |
| 3.4.1 |  | Power related |

Power related issues are the most fundamental reason for network failure. The main AC power flows into either an external or internal AC to DC transformer module within a device. The transformer provides correctly modulated DC current, which acts to power device circuits, connectors, ports and the blowers used for device cooling.

If a power related issue is suspected, a physical inspection of the power module is often carried out. Initially, with the power switch on, does the blower operate?

- If yes, the AC input checks out.
- If no, suspect the AC input, AC source, router circuit breaker, or the power supply cable.
- With the power switch on and system LEDs lit, do the fans operate?
- If no, suspect the fans.
- Does the system shut down after being on a short time?
- Suspect an environmentally induced shutdown.
- Check the environmental site requirements in device documentation and ensure that the chassis intake and exhaust vents are clear.
- Suspect a power supply failure, have other devices in the area powered down?
- System partially boots, but LEDs do not light.
- Suspect a 5-volt (V) power supply failure.

Distribution and core layer Cisco devices are generally modular in nature. This approach can follow through for the power supply(s) of these units. Often parallel and backup power supplies exist in the chassis to balance the power load and to provide uninterrupted operation in the event of failure of a single or multiple online power modules.

To help isolate a power subsystem problem, follow these steps:

- Check whether the power supply LED labeled GOOD is on or the LED labeled FAIL is on.
- If the LED labeled GOOD is off or if the LED labeled FAIL is on, take the following steps:

> Step 1
> Ensure that the power supply is flush with the back of the chassis.

> Step 2
> Unplug the power cord, loosen and reinstall the power supply, tighten the captive installation screws, and then plug in the power cord.

- If the LED labeled GOOD remains off, there might be a problem with the AC source or the power cable. Connect the power cord to another power source if one is available.
- If the LED labeled GOOD fails to light after the power supply is connected to a new power source, replace the power cord.

If this unit has more than one power cord, repeat the steps for each power supply.