e enterprise in recent years. As with OSPF, updates con- tinue to be made to IS-IS, including revisions that support IPv6 (IS-IS for IPv6) and Restart Signaling (another name for OSPF's Nonstop Forwarding). The Multitopol- ogy (MT) routing extension to IS-IS allows for routing between a set of indepen- dent IP topologies; it can be used for a variety of purposes, such as maintaining separate IGP routing domains for isolated multicast or IPv6 islands within the backbone, forcing a subset of an address space to follow a different topology, or adding an in-band management network on top of the original IGP topology. As with OSPF, IS-IS also has extensions that accommodate Traffic Engineering DiffServ and optical technologies. Exterior Routing Protocols Exterior routing occurs between autonomous sys- items and is of concern to service providers and other large or complex networks. As shown in Figure 8.6, gateway protocols are used within and between autono- mous systems. Whereas there may be many different interior routing schemes, a single exterior routing scheme manages the global Internet, and it is based on the exterior routing protocol BGP version 4 (BGP4). The basic routable element is the autonomous system. Routers determine the path for a data packet by calculating the number of hops between internetwork segments. Routers build routing tables and use these tables along with routing algorithms. Internet Basics Autonomous IGP/BGP System 1 IGP/BGP IGP/BGP IGP/BGP Autonomous IGP/BGP System 7 IGP/BGP Figure 8.6 The role of gateway protocols BGP, the core routing protocol of the Internet, works by maintaining a table of IP networks, or prefixes, that designate network reachability between autonomous systems. BGP makes use of a path-vector protocol, which means routing decisions are based on network policies or rules rather than on technical parameters, as is the case with distance-vector and link-state protocols. In BGP, the routing table maintains the autonomous systems that are crossed in order to reach the desti