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## Intelligent Power Distribution Units 

In racked configurations such as model 996 and 994, the rack power control is done through the Ethernet managed intelligent power distribution units (iPDUs). For rackless configurations such as model 993, the iPDUs are optional. 

An HMC communicates to these iPDUs via ethernet network and manages and monitors system power state, iPDU configuration, System AC power on/off, iPDU firmware update, iPDU health check/error and power usage reporting. 

The iPDUs network interfaces are also connected to the external ports of the ME to reach the black and gray network switches. They are distributed over the black and gray network. That means iPDUs that belong to one power domain (usually left side at the rear of the rack) connect to gray network switch and the iPDUs which belong to other power domain (usually right side at the rear of the rack) 

For Agility class racked models 996, there is the possibility to add an expansion rack model E96. Two cascaded switches are added to the base frame to connect the additional iPDUs of the expansion rack to the ME switches. 

One 1U 24-port ethernet switch is added for the black network and one 24-port is added for the gray network. Each of them has an uplink to the related ME switch port of their designated black or gray network. The 2U space required is already reserved at the bottom of the base rack. 

Figure 8-6 and Table 8-1 show how each port is used on the pair of DS8900F ME Ethernet switches. 

