Troubleshooting the M320 Cooling System

The M320 router cooling system comprises two front and one rear fan tray (see M320 Chassis Description). The front fan trays each contain four fans and are interchangeable. The rear fan tray contains seven fans and is not interchangeable with the front trays. The front fan trays cool the components installed in the front card cage (the FPCs, PICs, CIP, and midplane). The rear fan tray cools the components installed in the rear card cage (the Routing Engines, CBs, and the SIBs). Each power supply has two fans that cool that power supply. Figure 1 shows the airflow through the router.

Air filters for both the front and rear fan trays help keep dust and other particles from entering the cooling system. To function properly, the entire cooling system requires an unobstructed airflow and proper clearance around the site, as described in M320 Site Preparation Checklist Requirements.

Figure 1: Airflow Through the Chassis

During normal operation, the fans in each fan tray function at less than full speed. The CB constantly monitors the temperatures detected by sensors on the midplane and router components, adjusting the speed of the fans as necessary. If the router temperature exceeds the acceptable maximum, the CB turns off the power supplies. The following conditions automatically cause the fans to run at full speed and also trigger the indicated alarm:

• A fan fails (red alarm).
• One of the fan trays is removed (yellow alarm).
• The router temperature exceeds the “temperature warm” threshold (yellow alarm).
• The temperature of the router exceeds the maximum (“temperature hot”) threshold (red alarm and automatic shutdown of the power supplies).

To troubleshoot the fans, follow these guidelines:

• If the red alarm LED on the craft interface lights, find the source of the problem by looking at the display on the craft interface. The number of alarm conditions, as well as the source of each alarm, appears on the screen.
• Use the CLI to check the status of the fans. For example, you can issue the following command to get information about the source of an alarm condition:
  user@host> show chassis alarms

  For information about the alarms, see M320 Chassis and Interface Alarm Messages.

• Place your hand near the exhaust vents at the rear of the chassis to determine whether the fans are pushing air out of the chassis.
• If the OUTPUT OK LED on one or more of the power supplies (or the corresponding POWER OK LED on the craft interface) is not lit, check the power supply fans to see if they are operating.
• If all power supplies have failed, the system temperature might have exceeded the threshold, causing the system to shut down.
• If the display on the craft interface lists only one fan failure and the other fans are functioning normally, the fan is probably faulty and you need to replace the fan tray, as described in Replacing an M320 Fan Tray.