Intel QuickPath Interconnect replaces front-side bus
The Intel QuickPath Interconnect (QPI) is a point-to-point processor interconnect developed by Intel which replaced the front-side bus (FSB) in Xeon, Itanium, and certain desktop platforms starting in 2008. It increased the scalability and available bandwidth. Prior to the name's announcement, Intel referred to it as Common System Interface (CSI).  Earlier incarnations were known as Yet Another Protocol (YAP) and YAP+.
QPI 1.1 is a significantly revamped version introduced with Sandy Bridge-EP (Romley platform).QPI was replaced by Intel Ultra Path Interconnect (UPI) in Skylake-SP Xeon processors based on LGA 3647 socket.Background
Although sometimes called a "bus", QPI is a point-to-point interconnect. It was designed to compete with HyperTransport that had been used by Advanced Micro Devices (AMD) since around 2003. Intel developed QPI at its Massachusetts Microprocessor Design Center (MMDC) by members of what had been the Alpha Development Group, which Intel had acquired from Compaq and HP and in turn originally came from Digital Equipment Corporation (DEC).
Its development had been reported as early as 2004.Intel first delivered it for desktop processors in November 2008 on the Intel Core i7-9xx and X58 chipset.
It was released in Xeon processors code-named Nehalem in March 2009 and Itanium processors in February 2010 (code named Tukwila).It was supplanted by the Intel Ultra Path Interconnect starting in 2017 on the Xeon Skylake-SP platforms.