Juniper JUNOS OS 10.3 - SOFTWARE Manual page 477

Copyright © 2010, Juniper Networks, Inc.
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 72 on page 381 shows the effect on normal
operating power in N+1 and N+N configurations.
Table 72: Available Operating Power in N+1 and N+N Redundancy
Configurations
Number of Power Normal Operating Power in N+1
Supplies at n W Each Configuration
2 1 x (n W)
3 2 x (n W)
4 3 x (n W)
5 4 x (n W)
6 5 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
Chapter 18: Power Management Overview
Normal Operating Power in
N+N Configuration
1 x (n W)
2 x (n W)
2 x (n W)
3 x (n W)
3 x (n W)
381