Each domain is driven through a PCIe Gen3 Interconnect switch adapter. The two PCIe  switch cards provide a backup path for each other through the passive connection in the PCIe  I/O drawer backplane. During a PCIe fanout or cable failure, all 16 PCIe features in the two  domains can be driven through a single PCIe switch card (see Figure 2-25). Figure 2-25 Redundant I/O Interconnect 2.6.2 Enhanced drawer availability Note:  The PCIe Gen3 Interconnect (switch) adapter must be installed in the PCIe Drawer  to maintain the interconnect across I/O domains. If the adapter is removed, the I/O cards in  that domain (up to eight) become unavailable. To support Redundant I/O Interconnect (RII) between front to back domain pairs 0, 1 and 2, 3,  the two interconnects to each pair must be driven from two different PCIe fanouts. Normally,  each PCIe interconnect in a pair supports the eight features in its domain. In backup  operation mode, one PCIe interconnect supports all 16 features in the domain pair. With EDA, the effect of CPC drawer replacement is minimized. In a multiple CPC drawer  system, a single CPC drawer can be concurrently removed and reinstalled for an upgrade or  repair. Removing a CPC drawer without affecting the workload requires sufficient resources in  the remaining CPC drawer. Before removing the CPC drawer, the contents of the PUs and memory of the drawer must be  relocated. PUs must be available on the remaining CPC drawers to replace the deactivated  drawer. Also, sufficient redundant memory must be available if no degradation of applications  is allowed. To ensure that the CPC configuration supports removal of a CPC drawer with  minimal effect on the workload, consider the flexible memory option. Any CPC drawer can be  replaced, including the first CPC drawer that initially contains the HSA. Chapter 2. Central processor complex hardware components  69