Source and destination CAs directly connected. Refer to Figure 1-6 on page 17. The destination CA may be directly connected to the CA issuing the request packet. In this simple case, the request packet is transmitted over a single link and is received by the destination CA port. The port decodes the packet's DLID address field, determines that it is the targeted port, accepts the request packet, and processes the request.

Source and destination CAs aren't directly connected. Refer to Figure 1-3 on page 13. The destination CA may not be directly connected to the CA issuing the request packet. In this case, when the request packet is output onto the first link by the CA issuing the request, it arrives at a port on a switch or a router:

- Switch's Role. Refer to Figure 1-3 on page 13. Switches route packets within a local network referred to as a subnet. Using the packet's destination port address field (DLID), the switch performs a lookup in its Forwarding Table to determine through which of the switch's ports the packet must be transmitted to move it towards the destination CA port. The switch's Forwarding Table is set up by the configuration software (i.e., the SM) at startup time. The packet may have to transit one or more switches before it arrives at the destination CA port.

- Router's Role. Refer to Figure 1-3 on page 13. When the source and destination CAs reside in different subnets, the request packet contains a GRH (in addition to the LRH). As switches within a subnet move the request packet towards the destination port, the packet eventually arrives at a router port. The router uses the request packet's GRH:DGID port address to determine the subnet in which the destination CA resides. Specifically, it uses the packet's destination subnet ID field (i.e., the upper 64 bits of the 128-bit DGID address) to perform a lookup in its Routing Table, and then re-transmits the packet through the port indicated by the selected table entry. The router's Routing Table is set up by the configuration software at startup time. The packet may have to transit one or more routers before it arrives at the destination CA port.