incorporate numbers of hops, line speeds, amounts of traffic, and costs. With link-state routing, every node (router) receives a map, in the form of a graph, showing the connectivity of the network (i.e., which nodes are connected to which other nodes). Each node independently calculates the best next hop from itself to every possible node in the network. In this way, the routing table for each node is the collection of best next hops for that node. Unlike distance-vector routing, where each node shares its routing table with its neighbors, in link-state routing, the only information exchanged between the nodes is the information used to construct the connectivity maps. Link-state routing is more reliable, easier to debug, and less bandwidth intensive than distance-vector routing, but it is also more complex and more computer and memory intensive. Link-state algorithms are more efficient and create less network traffic than do distance-vector algorithms; this can be crucial in environments that involve multiple WAN links. OSPF, Intermediate System to Intermediate System (IS-IS), and NetWare Link Services Protocol (NLSP) are link-state routing protocols. Chapter 8 The Internet and IP Infrastructures Interior Routing Protocols Interior routing occurs within an autonomous system, which is a collection of routers under a single administrative authority that uses a common IGP for routing packets. Most of the common routing protocols, such as RIP, OSPF, and IS-IS, are interior routing protocols. The autonomous system number is a unique number that essentially identifies a portion of the Internet-usually owned or administered by a particular organization. Autonomous system numbers are managed and assigned by the RIRs (i.e., ARIN, APNIC, RIPE NCC, LACNIC, and AfriNIC). Exterior routing protocols, such as BGP, use autonomous system numbers to uniquely define borders between various net- works in the Internet. The basic routable element, the item on which the routing decision is based, is the IP network or subnetwork, or t