ocols and metrics are discussed in detail in Chapter 8, "The Internet and IP Infrastructures.") Routers can contain two types of network accessibility maps: static maps, which are manually configured and do not change, and dynamic maps, which can build routes automatically, as needed. A static router knows only its own table; it has no idea what the routing tables of its upstream neighbors look like, and it can- not communicate with its upstream neighbors. If a link goes down in a network that uses static routers, the network administrator has to manually reconfigure the Establishing Connections: Networking Modes and Switching Modes static routers' routing tables to take the downed trunk out of service. This reconfig- uration would require changes in the upstream routers, so technicians at those locations would then also have to include or accommodate the changes. Static routers are used only in tiny networks that do not change. A dynamic router, on the other hand, can communicate with its upstream neighbors, so if a change occurred to its routing table, it would forward that change SO that the other routers could also adjust their routing tables. Further- more, a dynamic router has a view of its own routing table, and it can also see those of its neighbors, or the entire network or routing area, depending on the pro- tocol. It therefore works much better in addressing the dynamic traffic patterns that are common in today's networks. Dynamic routers use the various routing pro- tocols mentioned earlier to communicate route topology changes to each other. Static routers do not use routing protocols. A router can use either static or dynamic routes, or both. As noted earlier in this chapter, there are two switching modes: circuit switch- ing and packet switching. Circuit switches are position based; that is, bits arrive in a certain position and are switched to a different position. The position to which bits are switched is determined by a combination of one or more of three dimensions: space (i.e., the interface 