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[MX] PEM low power and high power mode behavior on MX10008



Article ID: KB36017 KB Last Updated: 15 Jul 2020Version: 1.0

This article provides recommendations on PEM operational modes, and clears a few misunderstandings about PEM operational modes, PEM operation with mixed power modes, the Power Budgeting Algorithm, and the combined feature behavior.

Specifically, the article seeks to clarify a few misconceptions and also provide the correct behavior as opposed to that stated in MX10000 Power System.



The statement "If any power supply in the chassis is set to low power, all power supplies are also set to low power, regardless of their dip switch settings. This design is to prevent overloading of the power supply that is set to low power. See Table 2 for the settings for the dip switches." mentioned in MX10000 Power System is incorrect and should be disregarded.



The following output shows PEM usage:

labroot@router-re0> show chassis hardware models | match pem
PEM 0            Rev 02   740-069994   1F219110254       JNP10K-PWR-AC2
PEM 1            Rev 02   740-069994   1F219110333       JNP10K-PWR-AC2
PEM 2            Rev 02   740-069994   1F219110314       JNP10K-PWR-AC2
PEM 3            Rev 02   740-069994   1F219110128       JNP10K-PWR-AC2
PEM 4            Rev 02   740-069994   1F219110325       JNP10K-PWR-AC2
PEM 5            Rev 02   740-069994   1F219110235       JNP10K-PWR-AC2

Each PEM has three DIP switches accessible on its face panel. Each DIP switch has its own specific purpose as explained below:

  • The first and second DIP switch on a PEM is to turn the relative power feed On/OFF.

  • The third DIP switch on a PEM is related to Power Mode.

    • If the DIP is toggled ON, then the PEM is expected to work with High Power Mode.

    • If the DIP is toggled OFF, then the PEM is expected to work with Low Power Mode.

    • The power capacity of the input feed determines whether Low Power Mode or High Power Mode is used.

      • If the input feed has a maximum capacity of 30A, then High Power Mode is recommended. This is the default setting.

      • If the input power has a maximum capacity of 20A, then Low Power Mode is recommended.

    • Using Low Power Mode is recommended only when 30A power feed is unavailable and/or the system in a transitional period during which the location is being upgraded to fit the 30A requirement.

    • In Low Power Mode, the maximum power delivery of a PEM is reduced to almost 3000W (Low Power Mode) from 5500W (High Power Mode). 

    • Further, it is recommended to use all PEMs in High Power Mode or Low Power Mode, but not use a mix of both modes within the PEMs on a single router.

    • In extraordinary circumstances when the local power feeds are being upgraded from 20A feed to 30A feed, the router can support a mix of both modes between PEMs.

    • In this mixed power mode scenario, PEMs in Low Power Mode can handle a maximum load of 3000A and PEMs in High Power Mode can handle a maximum load of 5500W.

  • The internal power budgeting algorithm always tries to load balance power distribution between the PEMs by using the following table:

    • max load(Amp) difference of 3.0% between power supplies from 50% up to 100%

    • max load(Amp) difference of 5.0% between power supplies from 20% up to 50%

    • max load(Amp) difference of more than 5% allowed when the load is lower than 20%

  • Considering this table if the overall power consumption on the router is between 40% to 50%, then the load distribution algorithm tries to balance load within 5% between the PEMs operating in Low Power and High Power Modes.

  • In this case, a PEM operating in High Power Mode takes a load of around 50%, making it around 2800W, and due to the Power Budgeting Algorithm, the power in Low Power Mode reaches almost 93%.

  • So as the router reaches the higher side of 50% overall power consumption, the PEM operating in Low Power Mode will saturate and pose a risk of tripping the circuit breaker, and eventually moving the total load to PEMs that are operating in High Power Mode when the low power feed circuit breaker trips.

  • In short, PEMS on mixed power mode on a single router may operate without issue when there is overall low power consumption. However, it poses immense risk when the overall power consumption of the router increases.

  • Although mixed power mode is supported on MX10008 routers, it is not recommended by Juniper.

  • Each PEM is independent in its operating mode and changing the power mode on one PEM will not affect the operating power mode on the other PEM.

  • Max output power from a PEM when operating in Low Power Mode and connected only to a single feed is 2800W, as opposed to Juniper documentation's specification of 2700W.

For more information about hardware compatibility and support, refer to MX10000 Power System.

Each PEM has two input power feeds and both of them are connected to each and every PEM on the router as shown below:

DIP Settings on PEM0 (DIP1: ON, DIP2: ON, DIP:3 ON)
DIP Settings on PEM1 (DIP1: ON, DIP2: ON, DIP:3 OFF)
DIP Settings on PEM2 (DIP1: ON, DIP2: OFF, DIP:3 OFF)

labroot@router-re0> show chassis power detail                        
PEM 0:
  State:     Online
  Capacity:  5500 W (maximum 5500 W)
  AC input:  OK (Both feed expected, Both feed connected)
  Feed1:     AC input
  Feed2:     AC input
  DC output: 312 W (zone 0, 26 A at 12 V, 5% of capacity)

PEM 1:
  State:     Online
  Capacity:  3000 W (maximum 5500 W)
  AC input:  OK (Both feed expected, Both feed connected)
  Feed1:     AC input
  Feed2:     AC input
  DC output: 300 W (zone 0, 25 A at 12 V, 5% of capacity)

PEM 2:
  State:     Online
  Capacity:  2800 W (maximum 5500 W)
  AC input:  OK (Both feed expected, INP1 feed connected)
  Feed1:     AC input
  DC output: 216 W (zone 0, 18 A at 12 V, 3% of capacity)


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