Understanding Power Management on EX Series Switches
The power management feature for Juniper Networks EX8200 Ethernet Switches helps ensure that normal operation of the system is not disrupted because of insufficient power to the switch. It does so by employing a power budget policy.
Power management does the following in its power budget policy:
- Budgets power for each switch component that requires power. The amount that power management budgets for each component is the maximum power that component might consume. For example, for the fan tray, power management budgets the amount of power required to run the fans at their maximum speed setting.
- 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 Engine, 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
Power management powers line cards on or off based on the power priority of the slots they occupy:
- When a switch powers on, power management provides power to the line cards in the order of their slot priority, with line cards in high priority slots receiving power first. Thus if available power (including redundant power) is exhausted before all line cards receive power, higher priority cards are powered on while lower priority cards remain powered off.
- If the switch starts receiving insufficient power because of power supply failure or some other problem, power management powers off the line cards in reverse-priority order until power (including redundant power) is sufficient for the remaining cards. Thus higher priority line cards are more likely to retain power in power shortage conditions than are lower priority line cards.
- Power management responds to changes in power availability
and line card operating status by powering line cards on or off as
appropriate. For example, if you add a power supply, lower priority
cards that were powered off due to insufficient power are powered
on in priority order.
If you take a line card offline, power management no longer allocates power to it. If power to the switch is insufficient when you take a line card offline, power management allocates the freed power to a lower priority card that was offline due to lack of power and brings it online. Restarting a line card, however, does not affect the power allocated to it. Thus when power is insufficient, restarting a line card does not change its operating status or the operating status of other line cards.
Note: Because power management does not allocate power to an offline line card, a line card that has been taken offline in an EX8200 switch is not automatically brought online when you commit a configuration. You must explicitly bring the line card online with the request chassis fpc slot slot-number online command. This behavior differs from other platforms running Junos operating system (Junos OS), which automatically bring an offline FPC online when you commit a configuration.
The actual power priority of a slot is determined first by the slot’s assigned priority and second by the slot’s number. If more than one slot has the same assigned priority, power priority is determined by slot number, with the lowest-numbered slots receiving power first.
By default, all slots are assigned the lowest priority. You can assign a priority to a slot using the CLI. If you do not explicitly assign priorities to slots, the slots receive power in ascending order of slot numbers.
Because the purpose of power management is to ensure minimal system disruption when power is insufficient, slot power priority does not always determine which line cards receive power. In some cases, power management might provide power to a lower priority line card rather than a higher priority line card. For example:
- If power is insufficient for a line card in a higher priority slot but is sufficient for a line card in a lower priority slot, the lower priority slot receives the power. For example, if an 8-port SFP+ line card requiring 450 W is in a higher priority slot than a 48-port SFP line card requiring 330 W, the 48-port SFP line card receives the power if there is more than 330 W but less than 450 W available.
- In an operating switch that has insufficient power, power
management does not power off operating line cards to provide power
to a newly inserted line card or a line card that is brought online
after being offline, even if the line card has a higher priority than
the currently operating line cards.
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 you change the assigned power priority of line cards when there is insufficient power for all the line cards, power management does not power down line cards that had been receiving power because they are now a lower priority.
Power Supply Redundancy
By default, power management in EX8200 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 | 4 x (n W) | 3 x (n W) |
6 | 5 x (n W) | 3 x (n W) |
To compensate for the reduced normal operating power, power management reserves less power to the chassis in an N+N configuration than in an N+1 configuration. This reduction in reserved 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 reserved 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 reserved chassis power, 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. |
Power management automatically recalculates the redundant 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 redundant power. If you bring a fourth 2000 W power supply online, power management then allocates 4000 W to redundant power. If a power supply goes offline again, power management once again allocates 2000 W to redundant power.
When power is insufficient to meet the budgeted power requirements, power management raises alarms as follows:
- If all the line cards are receiving power but insufficient redundant power exists to maintain the configured N+1 or N+N power configuration, power management raises a minor (yellow) alarm. If this condition persists for 5 minutes, the alarm becomes a major (red) alarm.
- If one or more line cards are down because of insufficient power (including redundant power), power management raises a major (red) alarm.
Power management clears all alarms when sufficient power is available to meet normal operating and redundant power requirements.
