A site is a physical collection of computers in which communication between any two computers is fast and inexpensive. Site boundaries usually parallel the physical location of the computers (for example, all computers within a building). However, not all computers in a site have to run the same protocol, and computers in the same site may not be able to directly communicate with each other.
Sites are connected to other sites through communication links called site links. Inter-site routing is the process of sending messages between sites on these links. MSMQ calculates inter-site routing based on relative numbers that administrators assign to site links. These numbers, called site-link costs, represent the cost of communication of that link.
Although establishing site boundaries in MSMQ is fairly simple, additional factors should be considered for compatibility with the site object in future versions of Windows NT Server. For more information, see "Topology" in Chapter 6, "Deploying MSMQ."