Problem Solving Using a Subsystem Approach
The key to solving system problems is to try to isolate the difficulty to a specific subsystem. The first step in solving startup problems, for example, is to compare what the system is doing to what it should be doing.
Since a startup problem is usually attributable to a single component, it is more effective to isolate the problem to a specific subsystem, rather than trying to troubleshoot each system component.
For purposes of the troubleshooting procedures in this chapter, the Cisco 12008 will be regarded as consisting of the following subsystems:
•Power subsystem—The power subsystem consists of the following components:
—AC-input power supplies or DC-input power supplies—The Cisco 12008 can be configured to operate with either source AC power or source DC power.
Note An AC-input power supply and a DC-input power supply cannot be used together in the same Cisco 12008 chassis.
A Cisco 12008 can be configured to operate with either a single AC-input power supply or a single DC-input power supply. Adding a second power supply of the same type provides redundancy (backup power capability) and enables the power supplies to share the current load for the router.
—DC-DC converters—A DC-DC converter is incorporated into each of the cards that you can install in the upper card cage (the RP, CSCs, and line cards) or the lower card cage (the SFCs). The DC-DC converter on each card operates under control of its onboard MBus module.
The converter takes the –48 VDC supplied to the card through the backplane from the power supply(ies) and converts it into the +3.3 VDC and +5 VDC required by the card’s circuitry.
•Cooling subsystem—The cooling subsystem consists of two fan trays: one for the card cage and one for the power supply bays.
