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3.4 Cooling Systems

With the exception of the M5 and M10 routers, each Juniper Networks router incorporates redundancy in the cooling system. This section provides a general familiarity with the airflow through each model of the M-Series routers.


Never operate the router for more than one minute without the fan assembly installed! The function of the fans is to keep all components cooled to an optimal operating level. Running the unit without fans in place could void your warranty and limit any maintenance available to you. Check with your Juniper Networks representative for more information.

3.4.1 M5 and M10

The single fan assembly on either an M5 or M10 is on the left side of the router if you are facing the front of the chassis. The fan assembly contains four fans that pull air from the left to the right across the PICs. Juniper Networks suggests leaving six inches on either side of any installed M-Series router, as shown in Figure 3–11, to ensure adequate airflow.

Figure 3-11Figure 3–11 M5/M10 Airflow

3.4.2 M20

The M20 router has four fan assemblies—three on the front of the chassis and one in the rear. The three fans on the front left side of the chassis provide side-to-side cooling for the FPCs and for the SSB. The rear fan assembly is located to the right of the routing engine and provides cooling directly to the routing engine. All four fan assemblies plug directly into the router midplane. In addition, the power supplies on the M20 router have integrated fans, providing built-in power-supply cooling. The airflow model is shown in Figure 3–12.

Figure 3-12Figure 3–12 M20 Airflow

3.4.3 M40

The cooling system for the M40 router is a little more complex, consisting of three separate subsystems: the impellers, the integrated power-supply fans, and the triple fan assemblies.

The router contains two sets of redundant impellers, located at the top of the chassis and at the bottom of the card cage. These impellers pull air in through the front of the card cage and across the PFE, forcing the exhaust out the back of the chassis, thus cooling the PFE. You can see this process in Figure 3–13. The impellers are designed to run at less than full capacity unless a condition is detected, such as a rise in temperature, which would increase the cooling needs. At that point, the impellers can adjust the fan speed to meet the new requirements. An air filter protects the impeller from drawing in foreign objects that could damage the fans. It is a good idea to keep the air filter in place when the router is operational.

Figure 3-13Figure 3–13 M40 Airflow

A set of three load-sharing fan trays, located at the upper rear of the chassis, pull in air through a filter and intake at the front of the chassis to keep the routing engine cool. Because the fan trays are load-sharing, if one fan tray is removed, the others remaining will adjust to meet the current cooling requirement.

Finally, the power supplies on the M40 router have integrated cooling fans, just like those on the M20.

3.4.4 M160

The M160 router also uses impellers and fan trays to keep the system cool. A front cooling system uses an upper impeller that works with the fan tray (installed in front) to pull air through the front of the chassis, up through the card cage, and then sends the exhaust out the rear of the chassis.

The rear cooling system uses two impellers in the upper and lower part of the chassis to pull air in over the routing engine, SFM, MCS, and PCGs. As you can see in Figure 3–14, the air is drawn in through the front of the chassis, through the air intake cover, and the exhaust is sent out the rear of the chassis. The power supplies are cooled by the front and rear cooling systems and do not have integrated fans.

Figure 3-14Figure 3–14 M160 Airflow

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