During a scheduled power maintenance at the customer site, two power feeds—configured for redundancy—were supplying power to the MX chassis. As part of the maintenance procedure, the customer shut down one power feed at a time.

Unexpectedly, when one of the feeds was disconnected, the FRUs (RE, CB, and FPC) restarted. This behavior was not anticipated, given the redundant power feed setup.

labroot@raza-re0> show chassis power detail               

PEM 0:
 State:   Online
 DC input: Check (2 feed expected, 1 feed connected)
 DC input: 48.0 V input (57500 mV)
 Capacity: 1700 W (maximum 4100 W)
 DC output: 969 W (zone 0, 17 A at 57 V, 57% of capacity)

PEM 1:
 State:   Online
 DC input: Check (2 feed expected, 1 feed connected)
 DC input: 48.0 V input (57500 mV)
 Capacity: 1700 W (maximum 4100 W)
 DC output: 798 W (zone 1, 14 A at 57 V, 46% of capacity)

PEM 2:
 State:   Online
 DC input: Check (2 feed expected, 1 feed connected)
 DC input: 48.0 V input (57500 mV)
 Capacity: 1700 W (maximum 4100 W)
 DC output: 1083 W (zone 0, 19 A at 57 V, 63% of capacity)

PEM 3:
 State:   Online
 DC input: Check (2 feed expected, 1 feed connected)
 DC input: 48.0 V input (57000 mV)
 Capacity: 1700 W (maximum 4100 W)
 DC output: 1026 W (zone 1, 18 A at 57 V, 60% of capacity)

System:
 Zone 0:
   Capacity:     1700 W (maximum 4100 W)
   Allocated power:  1619 W (81 W remaining)
   Actual usage:   2052 W

 Zone 1:
   Capacity:     1700 W (maximum 4100 W)
   Allocated power:  1576 W (124 W remaining)
   Actual usage:   1824 W

Total system capacity: 3400 W (maximum 8200 W)

From the observed outputs, it’s noteworthy that each PEM is receiving input from both Feed A and Feed B. This non-standard configuration may be contributing to unexpected chassis shutdowns or triggering unusual alarms, potentially causing FRUs such as FPCs or REs to remain down.

Juniper’s recommended configuration is as follows:

DC power supplies in slots PEM0 and PEM1 should be powered by dedicated feeds from Feed A.
DC power supplies in slots PEM2 and PEM3 should be powered by dedicated feeds from Feed B.

This setup ensures proper A/B feed redundancy, which is commonly deployed to maintain system stability and prevent unexpected behavior during power events.

If the system is wired according to Juniper’s recommendation, it will continue to operate at full capacity even if one power feed is lost. The customer has been advised to rewire the router to align with Juniper’s recommended standard.