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[MX2020] Maintaining cooling and power with only one cooling zone



Article ID: KB28812 KB Last Updated: 10 Mar 2014Version: 1.0

This article describes power supply module (PSM)/fan operation in an MX2020 3D Universal Edge Router. MX2020 power and cooling are designed to be controlled by one zone or two. In most cases, MX2020 power and cooling are initially controlled by just one zone. As customer needs grow, PSM needs grow, eventually requiring control by two zones. Sometimes, for whatever reason, a customer might fail to install a second, needed PSM (such as in a zone with no line cards), which can lead to problems.


A scenario depicting a potential cooling/power problem on an MX2020 is shown below.

Minimum Number of FanTrays for MX2020 Chassis: 3

The critical FRUs are cooled down through both zone's cooling system, half-and-half. Click the Related Links below for more information.

Power Specification for FanTrays

FanTray#1 and FanTray#3 get power from the local zone only, whereas FanTray#0 and FanTray#2 can get power from both zones:

FT#3 - local power
FT#2 - redundant power - gets power from zone0 when there is no power in zone1
FT#1 - local power
FT#0 - redundant power - gets power from zone1 when there is no power in zone0

Hence, the expected result when the router has no PSMs in zone1 is as follows:

FT#3 - Check status
FT#2 - Normal - gets power from zone0
FT#1 - Normal
FT#0 - Normal

Junos Behavior in FanTray, Single-Fault Status

Junos considers it a single-fault FanTray; thus, the alarm shows that FanTray#3 is in Check status. Also, the CRAFT-LED for FT#3 is turned-off, while the FAN-LED is YELLOW.

Note: The CRAFT-LED is bicolor. As such, it cannot display YELLOW. It is simply turned OFF in this case. Also, both the CRAFT-LED and the FAN-LED for FT#3 or FT#1 remain GREEN even if the FanTray stops operating with no power. This bug is scheduled to be fixed in Junos 12.3R6 and later.

Junos Behavior in FanTray, Double-Fault Status

If a problem develops on FT#2 in this status (which indicates a *double-fault* in zone1), all critical FRUs in the middle of the box (especially SFMs) go down due to high-temperature within 1 minute:

Feb 7 14:18:40.269 MX2020-62 alarmd[1560]: %DAEMON-4: Alarm set: Fan color=RED, class=CHASSIS, reason=Fan Tray 2 Failure

Feb 7 14:20:09.837 MX2020-62 chassisd[68952]: %DAEMON-3-CHASSISD_FRU_HIGH_TEMP_CONDITION: SFB 2 SFB-XF2-Zone1 temperature 88 over 85 degrees C (fan/impeller failure detected)

Feb 7 14:20:14.580 MX2020-62 chassisd[68952]: %DAEMON-3-CHASSISD_FRU_HIGH_TEMP_CONDITION: SFB 2 SFB-XF2-Zone1 temperature 88 over 85 degrees C (fan/impeller failure detected)

Feb 7 14:20:49.106 MX2020-62 chassisd[68952]: %DAEMON-3-CHASSISD_TEMP_HOT_NOTICE: SFB 2 temperature of 93 degrees C is above limit (90 degrees)


Prevent a fault in a FanTray in a single zone. To do this, use at least one PSM in a zone, even if there are no line cards.

Note: A PSM can provide a miniumum 2100 W (the maximum is 2500 W), while a FanTray uses 1700 W.

Related Links

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