The outstanding reliability, availability, and serviceability (RAS) characteristics of IBM mainframe computers are among the features that gained the IBM eServere family its reputation as a leading platform for business-critical applications. The aim now is to further improve IBM System z9t RAS by introducing redundant I/O interconnect (RII) as a building block of enhanced book availability and recovery scenarios. RII provides a means of maintaining I/O connectivity during planned or unplanned outages in a way that is transparent to the operating system and customer applications. The mechanism that meets this requirement is the provision of an alternate path to the I/O cage, which provides highbandwidth I/O slots to enable a higher number of I/O ports per card. This paper discusses the I/O subsystem hardware and firmware aspects of RII.