Ravi Budruk, Don Anderson, and Tom Shanley

This chapter describes the general concepts of PCI Express transaction routing and the mechanisms used by a device in deciding whether to accept, forward, or reject a packet arriving at an ingress port. Because Data Link Layer Packets (DLLPs) and Physical Layer ordered set link traffic are never forwarded, the emphasis here is on Transaction Layer Packet (TLP) types and the three routing methods associated with them: address routing, ID routing, and implicit routing. Included is a summary of configuration methods used in PCI Express to set up PCI-compatible plug-and-play addressing within system IO and memory maps, as well as key elements in the PCI Express packet protocol used in making routing decisions.