Cisco Systems Cisco 12008 manual Troubleshooting Overview

Troubleshooting Overview

Each fan tray incorporates individual fans that draw +24 VDC from a DC-DC converter on the CSC that operates under control of its onboard MBus module. Both fan trays should begin to operate about two seconds after application of system power.

The fan trays incorporate a variable-speed feature, enabling the fans to run at a slower speed (when the internal chassis temperature remains within the normal operating range) or to run at a higher speed (when the internal temperature of the router exceeds a specified threshold). In the latter case, the voltage being delivered to the fans is increased, causing them to run at maximum speed to force a greater volume of air through the router.

In a noisy, air-conditioned environment, it may be difficult to hear whether or not the fan trays are operating. The first indication of a fan failure comes from the side-by-side fan tray status LEDs on the CSC faceplate (see Figure 1-14).

If the LED on the left is on (amber), a fan failure has occurred in the card cage fan tray. If the LED on the right is on (amber), a fan failure has occurred in the power supply fan tray.

Each fan in a fan tray is monitored separately for failure. A failed fan is not shut off in the usual sense; instead, a current-limiting feature in the faulty fan prevents it from interfering with the operation of other fans. The most common fan failure is that the fan just stops running, but continues to draw current.

If a fan in the card cage fan tray or the power supply fan tray fails, the CSC increases the voltage being delivered to the fans, causing them to run at maximum speed to compensate for the failed fan.

You can make a gross assessment of fan tray operation by placing your hand near the exhaust vents at the top rear of the router enclosure. A reduced flow of air from the card cage exhaust vent or the power supply exhaust vent may indicate that one or more fans on a given fan tray have failed. However, this method of checking fan operation is neither definitive nor reliable.

If you determine that a fan tray is not operating properly, you should immediately consider replacing the faulty fan tray to guard against an overheating condition in the router that could approach the shutdown threshold.

No operating adjustments can be made for either fan tray. An individual fan is either operating normally or it is not operating at all.

Troubleshooting the Installation 5-5