This chapter discussed the basics of router networks, including the following:
The internetworking world is focusing on a unified Layer 3 protocol called IP. IP is particularly well suited for use in an internetworking environment, where several disparate networks need to be connected. Routers (historically called gateways) allow the IP data to be switched between interconnected networks via the Network Layer (Layer 3) part of a packet.
Scalable routing over large internetworks can be facilitated by imposing hierarchy on the overall design. At the highest level we have the Autonomous System (AS), and this is broken down successively into smaller, manageable domains and areas until we reach the subnet. Hierarchy reduces traffic, reduces load on routers, and assists in minimizing the scope of router misbehavior.
Routing protocols are broadly divided into two groups: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). IGPs are broadly divided into two groups, based on the algorithms they use: distance vector and link state. Link-state algorithms scale better than distance-vector algorithms and are generally associated with more sophisticated, modern routing protocols such as OSPF and IS-IS.
Several mechanisms are available to allow end systems to operate within a routed environment, including Proxy ARP, passive RIP, ICMP Router Discovery (IRDP), and static gateway configuration. Possibly the best dynamic technique available today is ICMP Router Discovery.
The metrics used by routing protocols vary widely, from simple hops to fine-grained combinatorial metrics (incorporating such features as cost, delay, throughput, etc.). Metrics are fundamental for topology analysis and the creation of effective forwarding databases.
Router architecture has a fundamental effect on performance and is improving all the time. The first generation of multiprocessor architectures has been deployed, and advances in scheduling, lookup, and queuing algorithms have decreased router switching times considerably over recent years.
Route lookup time is fundamental to the scalability of routers in the backbone domain. As the number of backbone routes increases, faster algorithms, better data structures, and techniques such as lookup avoidance are increasingly under scrutiny.
 T. Kenyon, High-Performance Network Design: Design Techniques and Tools (Woburn, MA: Digital Press, 2001).
 Border Gateway Protocol 3 (BGP-3), RFC 1267, October 1991.
 A Framework for QoS-based Routing in the Internet, RFC 2386, August 1998.
 www.cisco.com, Cisco home page.
 Exterior Gateway Protocol (EGP), RFC 827, October 1982.
 Exterior Gateway Protocol Formal Specification, RFC 904, April 1984.
 S. Deering, ICMP Router Discovery Messages, RFC 1256, September 1991.