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CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 58

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 58 is the Cisco approved textbook to use alongside version 3.1 of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | chpt_10.fm Page 539 Tuesday May 27 2003 8 57 AM WAN Routers 539 More Information Roles of the Router in WANs Continued Autonomous system An autonomous system AS is a network or internetwork either LAN or WAN that is relatively self-contained. It is administered by a single person or organization features a single routed protocol has an address architecture and usually involves just one routing protocol. An autonomous system can support connections to other autonomous systems owned and operated by the same organization. Alternatively an AS might have connections to other networks such as the Internet yet it retains autonomy of operation. This term is usually used in conjunction with specific routing protocols such as Open Shortest Path First OSPF or Border Gateway Protocol BGP that enable a network to be carved into numbered subsections. Given these definitions it is possible to better define the functional classes of routers. An interior router is one that can be used by end systems in a network to access other end systems within the same network. The interior router supports no connections to any other network. Figure 10-11 illustrates a small network and identifies those devices that function as interior routers. Figure 10-11 Interior Routers in a Network Interior Routers Private Network A Private Network B An exterior router is one that lies beyond the boundaries of any given network. Figure 10-12 although not pretending to depict the Internet s actual topology presents a highly simplified Internet topology that is solely intended to demonstrate what an exterior router is. continues chpt_10.fm Page 540 Tuesday May 27 2003 8 57 AM 540 Chapter 10 WANs and Routers More Information Roles of the Router in WANs Continued Figure 10-12 Exterior Routers Private Network A Private Network B The last functional class of router is the border router. As the name implies border routers interconnect a network with other networks. It is important to note that a single entity .