some paths and routers. In dynamic routing, the routing table changes as network conditions change. A dynamic router communicates with other routers SO that they can all update their tables to reflect any changes. Routing Protocols There are two broad types of routing protocols: interior and exterior. Interior rout- ing protocols are typically used only in the routers of an enterprise's intranet (i.e., its internal network). Interior routing protocols include Routing Information Pro- tocol (RIP) and Open Shortest Path First (OSPF). Exterior protocols are typically used for routers located in the Internet, which is composed of many different pro- viders. Another application for exterior protocols is where routers are connecting systems between different organizations. Whereas there may be many different interior routing schemes, a single exterior routing system manages the whole glo- bal Internet, called Border Gateway Protocol 4 (BGP4). (Routing protocols are dis- cussed in detail in Chapter 8.) Hierarchical Networks Routers, when built into an enterprise internetwork, create a hierarchical network (i.e., subnetworks interconnected by routers). They control traffic flow through segmentation, but this can degrade network performance because of delays, and it adds complexity to the overall network configuration. Hierarchical networks are typically used at the edge of a network to interconnect LANs or to provide WAN connectivity to remote LANs. Again, within the customer premises, the simpler bridge-based flat networks were traditionally used. However, most campus networks today are primarily switched, and large campuses use routers to segment their broadcast domains. As traffic levels keep growing and LANs keep getting busier, both the edge and the core network are becoming loaded, resulting in network slowness and unacceptable delays. Routers use a hierarchical addressing scheme, whereby the address includes both the network address and the node address. Routers operate at Layer 3, SO they are networkable-you can 