PCI-PCI bridges are special PCI devices which glue the PCI busses of the system together. Simple systems have a single PCI bus but there is an electrical limit on the number of PCI devices that a single PCI bus can support. Using PCI-PCI bridges to add more PCI busses allows the system to support many more PCI devices. This is particularly important for a high performance server. Of course, Linux fully supports the use of PCI-PCI bridges.