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Cisco ASR 9000 Series Aggregation Services Router Overview and Reference Guide
OL-17501-09
Chapter 3 High Availability and Redundant Operation
Cooling System Redundancy
The Cisco ASR 9904 Router has a single fan tray located at the left side of the chassis and is accessible
from the rear. The fan tray has 12 fans arranged in three groups of four fans each. Two fans in each group
share a fan controller. The power supplied to the fan controller is 1:3 protected. A single fan failure has
no impact on air flow because the other eleven fans will compensate for it. If a fan controller fails, there
is a possibility of up to two fans failing; however, the design always has two fans operating to
compensate for the air speed.
In the Cisco ASR 9922 Router, the two top fan trays are located between the top and middle card cages,
while the two bottom fan trays are located between the middle and bottom card cages. In the Cisco ASR
9912 Router, the two fan trays are located above the line card cage. Each fan tray has 12 fans arranged
in three groups of four fans each. Two fans of each group share a fan controller. The power supplied to
the fan controller is 1:3 protected. A single fan failure has no impact on air flow because the other 11
fans will compensate for it. If the fan controller fails, there is a possibility of up to two fans failing;
however, the design always has two fans operating in a row (three rows of fans) to compensate for the
air speed.
Caution If only one fan tray is installed in the system, one single point of failure does not cause all fans to stop.
However, the system cannot operate without a fan tray. The system shuts itself off if all fan trays are
removed and the system crosses the Shutdown Temperature Threshold (STT).
Cooling Failure Alarm
Temperature sensors are installed in all cards and fan trays. These sensors detect and report any fan
failure or high temperature condition, and raise an alarm. Fan failure can be a fan stopping, fan controller
failure, power failure, or a failure of a communication link to the RSP/RP card.
Every card has temperature measurement points in the hottest expected area to clearly indicate a cooling
failure. The line cards have two sensors, one at the inlet and one near the hottest devices on the card. The
RSP/RP card also has two sensors.
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