Topology Is Unknown At Startup

Refer to Figure 19-2 on page 714. When the system is first powered up, the configuration software has not yet scanned the PCI Express fabric to discover the machine topology and how the fabric is populated. The configuration software is only aware of the existence of the Host/PCI bridge within the Root Complex and that bus number 0 is directly connected to the downstream (i.e., secondary) side of the bridge.

It has not yet scanned bus 0 and therefore does not yet know how many PCI Express ports are implemented on the Root Complex. The process of scanning the PCI Express fabric to discover its topology is referred to as the enumeration process.