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To support Redundant I/O Interconnect (RII) between domain pair 0 and 1, 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. 

Note:  The PCIe Gen3 Interconnect (switch) adapter must be installed in the PCIe+ I/O drawer to maintain the interconnect across I/O domains. If the adapter is removed (for a service operation), the I/O cards in that domain (up to eight) become unavailable. 

## 2.7.2 CPC drawer upgrades 

All fanouts that are used for I/O and coupling links are rebalanced if the upgrade does not involve adding components inside the CPC drawer. If MES involves extra components, such as PU SCMs and Memory DIMMs, the change is disruptive. 

When a z14 ZR1 is ordered, the PUs are characterized according to their intended usage. The PUs can be ordered as any of the following items: 

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A minimum of one PU that is characterized as a CP, IFL, or ICF is required per system. The maximum number of characterizable PUs is 30 (any combination of 6 CPs, 30 IFLs, 30 ICF, two more SAP$^{10}$s, and 12 ZIIPs). The maximum number of zIIPs is up to twice the number of PU that are characterized as CP. 

Remainder:  Not all PUs on a model must be characterized. 

