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Understanding Power Management on EX Series Switches

SUMMARY Power management on EX series switches helps prevent your switch from being disrupted if there's not enough power for all the switch components.

The power management feature for Juniper Networks Ethernet Switches helps ensure that normal operation of the system is not disrupted because of insufficient power to the switch. For example:

  • Power management ensures that operating line cards continue to receive power if a user installs a new line card in an operating switch when power is insufficient for both the new and existing line cards.

  • Power management reserves a certain amount of power to power supply redundancy, so that if a power supply fails, the switch can continue to operate normally. If power management must use some of this reserved power to provide power to switch components, it raises an alarm to indicate that power supply redundancy no longer exists and that normal operations might be disrupted if a power supply fails.

  • If power supply failure requires power management to power down some components, it does so gracefully by powering down line cards and PoE ports in the order specified by the user.

Power management manages power to switch components by employing a power budget policy. In its power budget policy, power management:

  • Budgets power for each installed switch component that requires power. With the exception of PoE power for line cards that support PoE, the amount that power management budgets for each component is the maximum power that component might consume under worst case operating conditions. For example, for the fan tray, power management budgets the amount of power required to run the fans at their maximum speed setting, even if the current fan speed is much lower.

  • Reserves a set amount of power for power supply redundancy. In its default configuration, power management manages the switch for N+1 power redundancy, which ensures uninterrupted system operation if one power supply fails. For example, if a switch has four online 3000 W power supplies, power management reserves 3000 W in its power budget policy for redundancy. It allocates the remaining 9000 W to normal operating power.

  • Specifies the rules under which components receive power. These rules are designed to ensure the least disruption to switch operation under conditions of insufficient power. For example, power management provides power to core system components, such as the Routing Engines, before it provides power to line cards.

You can configure certain aspects of power management’s budget policy, specifically:

  • The power priority of individual line cards. By assigning different power priorities to the line cards, you can determine which line cards are more likely to receive power in the event of insufficient power.

  • The power redundancy configuration. The default power redundancy configuration is N+1; you can optionally configure N+N. For example, if you have deployed two independent AC power feeds to the switch, configure N+N redundancy. When you configure power management for N+N redundancy, it reserves the appropriate amount of power in its power budget and reports insufficient power conditions accordingly.

These configurable items are discussed further in:

Power Priority of Line Cards

The power priority of line cards determines:

  • The order in which line cards are allocated power

  • The order in which line cards that support PoE are allocated power for PoE

  • How power is reallocated in cases of changes in power availability or demand in an operating switch

Note:

On EX6200 switches, the four 10-Gigabit Ethernet SFP+ uplink ports on a Switch Fabric and Routing Engine (SRE) module are treated like a line card in the power budget.

This section covers:

How a Line Card’s Power Priority Is Determined

Using the CLI, you can assign a explicit power priority to a line-card slot. If more than one slot has the same assigned priority, the power priority is determined by slot number, with the lowest-numbered slots receiving power first.

By default, all slots in an EX8200 switch are assigned the lowest priority. Thus if you do not explicitly assign priorities to slots, power priority is determined by slot number, with slot 0 having the highest priority.

In an EX6200 switch, all slots are assigned the lowest priority, except for the slots containing an SRE module. Slots containing an SRE module are automatically assigned the highest priority. This means that the line cards that represent the 10-Gigabit Ethernet SFP+ ports on SRE modules have the highest priority among the line cards.

Line Card Priority and Line Card Power

When an EX6200 or EX8200 switch is powered on, power management allocates power to components according to its power budget policy. After power management has allocated power to the base chassis components, it allocates the remaining available power to the line cards. It powers on the line cards in priority order until all line cards are powered on or the available power (including reserved power, if necessary) is exhausted. Thus if available power is exhausted before all line cards receive power, higher-priority cards are powered on while lower-priority cards remain powered off.

A lower-priority card might receive power while a higher-priority card does not if the remaining available power is sufficient to power on the lower-priority card but not the higher-priority card. For example, if a line card requiring 450 W is in a higher-priority slot than line card requiring 330 W, the line card requiring 330 W receives the power if there is less than 450 W but more than 330 W remaining in the power budget.

Line cards that have been administratively taken offline are not allocated power.

Note:

Because power management does not allocate power to a line card that has been administratively taken offline, a line card that has been taken offline in an EX6200 or EX8200 switch is not automatically brought online when you commit a configuration. You must explicitly use the request chassis fpc slot slot-number online command to bring a line card online that was taken offline previously. This behavior differs from other platforms running Juniper Networks Junos operating system (Junos OS), which automatically bring an offline FPC online when you commit a configuration.

If power management cannot power on a line card because of insufficient power, it raises a major (red) alarm.

Line Card Priority and PoE Power

After all line cards have been powered on, power management allocates any remaining available power, including reserved power, to the PoE power budgets of line cards that have PoE ports. Power management allocates PoE power to line cards in the order of power priority. If enough power is available, a line card receives its full PoE power budget before power management allocates PoE power to the next highest-priority line card. If not enough power is available, a line card receives partial PoE power and lower-priority line cards receive no PoE power.

If power management is unable to allocate enough power to meet the PoE power budget for a line card, it logs a message to the system log.

The default PoE power budget for a line card is the amount of power needed to supply the maximum supported power to all PoE ports. In cases where powered devices do not require the maximum power or in which some PoE ports are not used for powered devices, you can configure a smaller PoE power budget for a line card. By configuring a smaller PoE power budget, you make more power available for the PoE power budgets of lower-priority line cards.

You can also configure the power priority of the PoE ports on a line card. If power management is unable to allocate enough power to a line card to meet its PoE power budget, the line card PoE controller will turn off power to PoE ports in reverse priority order as required to meet the reduced power allocation.

See Configuring PoE Interfaces on EX Series Switches for more information on how to configure the PoE power budget for a line card and how to configure PoE port priorities.

Line Card Priority and Changes in the Power Budget

In an operating switch, power management dynamically reallocates power in response to changes in power availability or demand or changes in line card priority. Power management uses line card priority to determine how to reallocate power in response to the following events:

  • A power supply fails, is removed, or is taken offline:

    • If power is insufficient to meet the PoE power allocations of all PoE line cards, power management deallocates PoE power from the line cards in reverse priority order until power is sufficient to meet the remaining PoE power allocations.

    • If power is insufficient to meet the base (non-PoE) power requirements of all the line cards, all PoE power is deallocated. If, after the deallocation of PoE power, power is still not sufficient, power management turns off line cards in reverse priority order until power is sufficient for the remaining line cards.

  • A new line card is inserted or a line card is brought online:

    • If the line card supports PoE and there is insufficient power to meet its PoE power budget, PoE power is reallocated from lower-priority line cards. If not enough PoE power can be reallocated from lower-priority line cards, the new line card receives a partial PoE power allocation.

    • If there is insufficient power to power on the new line card, PoE power is removed from PoE line cards in reverse priority order until the new line card can be powered on.

    • If the removal of all PoE power is insufficient to free up enough power to power on the line card, the line card remains powered off and the PoE line cards continue to receive their PoE power allocations. To minimize disruption on an operating switch, lower-priority line cards are not turned off to provide power to the new line card. However, if you restart the switch, power management reruns the current power budget policy and powers line cards on or off based on their priority. As a result, line cards receive power strictly by priority order and previously operating line cards might no longer receive power.

  • A new power supply is brought online:

    • Any line cards that were powered off because of insufficient power are powered on in priority order.

    • After all line cards are powered on, remaining power is allocated to the PoE power budgets of line cards in priority order.

  • A line card is removed or taken offline, freeing up power:

    • Any line cards that were powered down because of insufficient power are powered on in priority order.

    • After all line cards are powered on, any remaining power is allocated to the PoE power budgets of line cards in priority order.

  • A user changes the assigned power priority of one or more line cards when power is insufficient to meet the power budget:

    • PoE power to the line cards is reallocated based on the new power priorities.

    • Base power allocation to the line cards is not changed—in other words, power management does not power down line cards that had been receiving power because they are now a lower priority. However, if you restart the switch, power management reruns the current power budget policy and powers line cards on or off based on their priority. As a result, line cards receive power strictly by priority order and previously operating line cards might no longer receive power.

If, because of insufficient power, power management reduces or eliminates the PoE power budget for a line card, it logs a message to the system log. If power management must power down a line card because of insufficient power, it raises a major (red) alarm.

Power Supply Redundancy

By default, power management in EX Series switches is configured to manage the power supplies for N+1 redundancy, in which one power supply is held in reserve for backup if one of the other power supplies is removed or fails.

You can configure power management to manage the power supplies for N+N redundancy. In N+N redundancy, power management holds N power supplies in reserve for backup. For example, if your switch has six power supplies and you configure N+N redundancy, power management makes three power supplies available for normal operating power and reserves three power supplies for redundancy (3+3). If you have an odd number of power supplies, power management allocates one more power supply to normal operating power than to redundant power. For example, if you have five power supplies, the N+N configuration is 3+2.

Given the same number of power supplies, an N+N configuration usually provides less normal operating power than an N+1 configuration because the N+N configuration holds more power in reserve for backup. Table 1 shows the effect on normal operating power in N+1 and N+N configurations.

Table 1: Available Operating Power in N+1 and N+N Redundancy Configurations

Number of Power Supplies at n W Each

Normal Operating Power in N+1 Configuration

Normal Operating Power in N+N Configuration

2

1 x (n W)

1 x (n W)

3

2 x (n W)

2 x (n W)

4

3 x (n W)

2 x (n W)

5 (EX8200 switches only)

4 x (n W)

3 x (n W)

6 (EX8200 switches only)

5 x (n W)

3 x (n W)

To compensate for the reduced normal operating power, power management on EX8200 switches allocates less power to the chassis in an N+N configuration than in an N+1 configuration. This reduction in allocated chassis power allows a switch in an N+N configuration to power more line cards than it could without the reduction. For the EX8208 switch, the power allocated for the chassis is reduced to 1200 W from 1600 W; for the EX8216 switch, it is reduced to 1800 W from 2400 W.

Note:

To achieve the reduction in allocated chassis power in an EX8200 switch, power management reduces the maximum fan speed to 60 percent in an N+N configuration from 80 percent in an N+1 configuration. Because the maximum fan speed is reduced, it is possible that a line card that overheats would be shut down sooner in an N+N configuration than in an N+1 configuration.

On EX6200 switches, the same amount of power is allocated for the chassis in N+N configurations as in N+1 configurations.

Power management automatically recalculates the reserved power and normal operating power as power supplies go online or offline. For example, if you have an N+N configuration with three online 2000 W power supplies, power management allocates 2000 W to reserved power. If you bring a fourth 2000 W power supply online, power management then allocates 4000 W to reserved power. If a power supply goes offline again, power management once again allocates 2000 W to reserved power.

When power is insufficient to meet the budgeted power requirements, power management raises alarms as follows:

  • A minor (yellow) alarm is raised when insufficient power exists to maintain the configured N+1 or N+N power reserves, but all line cards are still receiving their base and PoE power allocations. If this condition persists for 5 minutes, the alarm becomes a major (red) alarm. Even though operation of the switch is unaffected in this condition, you should remedy it as quickly as possible because a power supply failure might cause a disruption in switch operation.

  • A major (red) alarm is raised when insufficient power exists to provide all the line cards with their base and PoE power allocations. One or more PoE ports might be down or one or more line cards might be down.

Power management clears all alarms when sufficient power is available to meet normal operating and reserved power requirements.

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