Identifying the Classes of Routing Protocols 699 Z) are connected by three link-state routers. Network discovery for link-state routing uses the following processes: 1. Routers exchange LSAs with each other. Each router begins with directly con- nected networks for which it has direct, firsthand information. 2. Each router, in parallel with the others, constructs a topological database consist- ing of all the LSAs from the internetwork. 3. The SPF algorithm computes network reachability. The router constructs this logical topology as a tree, with itself as root, consisting of all possible paths to each network in the link-state protocol internetwork. It then sorts these paths by shortest path first (SPF). 4. The router lists its best paths and the ports to these destination networks in the routing table. It also maintains other databases of topology elements and status details. Figure 15-21 Link-State Algorithm Shortest Path Calculations When a router becomes aware of a link-state topology change, it forwards the infor- mation so that all other routers can use it for updates. This involves sending common routing information to all routers in the internetwork. To achieve convergence, each router keeps track of its neighbor routers, the router name, interface status, and the cost of the link to the neighbor. The router constructs an LSA packet that lists this infor- mation along with new neighbors, changes in link costs, and links that are no longer valid. The LSA packet then is sent out so that all other routers receive it. Figure 15-22 shows an example of link-state topology changes. A WXY Z BC 0 0 W X Routing Table 0 0 X Y Routing Table 0 0 Y Z Routing Table Topological Database SPF Tree A Routing Table SPF Topological Database SPF Tree B Routing Table SPF Topological Database SPF Tree C Routing Table SPF 1102.book Page 699 Tuesday, May 20, 2003 2:53 PM 700 Chapter 15: Routing and Routing Protocols Figure 15-22 Link-State Topology Changes When the router receives an LSA packet, the database is updated with the most recent information. The accumulated data is used to compute a map of the internetwork, and the SPF algorithm calculates the shortest path to other networks. Each time an LSA packet changes the link-state database, SPF recalculates the best paths and updates the routing table. Every router takes the topology change into account to determine the shortest path to use for packet routing. Three Link-State Concerns Running link-state protocols brings up three primary concerns: ■ Processor overhead ■ Memory requirements ■ Bandwidth consumption Routers running link-state protocols require more memory and perform more process- ing than distance vector routing protocols. Routers must have enough memory to hold all the information from the various databases, the topology tree, and the routing table, as shown in Figure 15-23. Initial link-state packet flooding consumes bandwidth. Dur- ing the initial discovery process, all routers using link-state routing protocols send LSA packets to all other routers. This action floods the internetwork and temporarily reduces the bandwidth available for routed traffic carrying user data. After this initial flooding, link-state routing protocols generally require only minimal bandwidth to send infre- quent or event-triggered LSA packets reflecting topology changes. 1102.book Page 700 Tuesday, May 20, 2003 2:53 PM Configuring a Router to Use Routing Protocols 701 Figure 15-23 Link State Concerns Hybrid Routing Protocol Features A third type of routing protocol, balanced hybrid routing, combines aspects of both distance vector and link-state routing protocols. Balanced hybrid routing protocols use distance vectors with more accurate metrics to determine the best paths to destination networks. However, they differ from most distance vector protocols by using topology changes to trigger routing database updates instead of periodic updates. Like link-state protocols, the balanced hybrid routing protocol converges rapidly. How- ever, it differs from distance vector and link-state protocols by using less bandwidth, memory, and processor overhead. Examples of hybrid protocols are Intermediate System-to-Intermediate System (IS-IS) and Enhanced Interior Gateway Routing Protocol (EIGRP). Configuring a Router to Use Routing Protocols To enable an IP routing protocol on a router, both global and interface parameters must be set. Global tasks include selecting a routing protocol, such as RIP, IGRP, EIGRP, or OSPF. The major task in the routing configuration mode is to indicate IP 1102.book Page 701 Tuesday, May 20, 2003 2:53 PM 702 Chapter 15: Routing and Routing Protocols network numbers. Dynamic routing uses broadcasts and multicasts to communicate with other routers. The routing metric helps routers find the best path to each network or subnet. The router command starts a routing process. The syntax for the router command is as follows: Router(config)#router protocol { options } where ■ protocol is either RIP, IGRP, or EIGRP. ■ options refers to autonomous system number such as those used by IGRP and EIGRP. The network command is necessary because it enables the routing process to identify the interfaces that participate in the sending and receiving of routing updates. The syn- tax for the network command is as follows: Router(config-router)#network network number where network number is the IP network number of a directly connected network. For RIP and IGRP, the network numbers must be based on the network class addresses, not subnet addresses or individual host addresses. Major network addresses are limited to Class A, B, and C network numbers. At the Internet layer of the TCP/IP suite of protocols, a router can use an IP routing protocol to accomplish routing through the implementation of a specific routing algorithm. Examples of IP routing protocols include the following, as shown in Figure 15-24: ■ Routing Information Protocol (RIP)—A distance vector interior routing protocol ■ Interior Gateway Routing Protocol (IGRP)—A distance vector interior routing protocol developed by Cisco ■ Open Shortest Path First (OSPF)—A link-state interior routing protocol ■ Enhanced Interior Gateway Routing Protocol (EIGRP)—A balanced hybrid interior routing protocol developed by Cisco ■ Border Gateway Protocol (BGP)—An exterior routing protocol 1102.book Page 702 Tuesday, May 20, 2003 2:53 PM Autonomous Systems and IGP vs. EGP 703 Figure 15-24 Routing Protocols Autonomous Systems and IGP vs. EGP An interior gateway protocol (IGP) is designed for use in a network controlled or administered by a single organization. An IGP is designed to find the best path through the network. In other words, the metric and how that metric is used is the most impor- tant element in an IGP. An exterior gateway protocol (EGP) is designed for use between networks that are controlled by two different organizations. EGPs typically ARE used between Internet service providers (ISPs) or between a company and an ISP. For example, a company would run BGP, an EGP, between one of its routers and a router inside an ISP. IP EGPs require the following three sets of information before routing can begin: ■ A list of neighbor routers to exchange routing information with ■ A list of networks to advertise as directly reachable ■ The autonomous system number of the local router An EGP must isolate autonomous systems. Because autonomous systems are managed by different administrations, networks must have a protocol to communicate between different systems. Figure 15-25 shows an autonomous system. 1102.book Page 703 Tuesday, May 20, 2003 2:53 PM 704 Chapter 15: Routing and Routing Protocols Figure 15-25 Autonomous System Every autonomous system is assigned a 16-bit identifying number by the American Registry of Internet Numbers (ARIN) or an ARIN provider. Routing protocols such as Cisco IGRP and EIGRP require that a unique, autonomous system number be assigned. Summary In this chapter, you learned the following points: ■ A router will not forward a packet without a route to a destination network. ■ Network administrators manually configure static routes. ■ Default routes are special static routes that provide routers with gateways of last resort. ■ Static and default routes are configured using the ip route command. ■ Static and default route configuration can be verified using the show ip route, ping, and traceroute commands. ■ Three types of routing protocols exist: — Distance vector — Link-state — Balanced hybrid ■ An autonomous system (AS) is a collection of networks under a common admin- istration and sharing a common routing strategy. To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. 1102.book Page 704 Tuesday, May 20, 2003 2:53 PM Key Terms 705 Key Terms administrative distance A rating that shows trustworthiness of a routing information source. This value is shown as a numeric value between 0 and 255. The higher the value is, the lower the trustworthiness rating is. autonomous system A collection of networks under a single administrative domain. balanced hybrid routing protocol Routing protocols that utilize elements of distance vector and link-state routing protocols. distance vector routing protocol A class of routing algorithms that iterate on the number of hops in a route to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms. Also called a Bellman- Ford routing algorithm. dynamic routing Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing. Requires that a routing protocol be run between routers. exterior gateway protocol (EGP) A routing protocol designed for use between networks that are controlled by two different organizations. interior gateway protocol (IGP) A routing protocol that is designed for use in a network controlled or administered by a single organization. link-state advertisements (LSAs) Small packets of routing information that are sent between routers. link-state routing protocol A routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching each of its neighbors to all nodes in the internetwork. Link-state algorithms create a consistent view of the network and are therefore not prone to routing loops. However, they achieve this at the cost of rela- tively greater computational difficulty and more widespread traffic than do distance vector routing algorithms. load sharing When dynamic routing protocols direct traffic from the same session over different paths in a network for better performance. metric An algorithm that generates a number for each path through the network. Typically, the smaller the metric number is, the better the path is. routed protocol A protocol that can be routed by a router. A router must be capable of interpreting the logical internetwork as specified by that routed protocol. Examples of routed protocols are AppleTalk, IPX, and IP. 1102.book Page 705 Tuesday, May 20, 2003 2:53 PM 706 Chapter 15: Routing and Routing Protocols routing protocol A protocol that accomplishes routing through the implementation of a specific routing algorithm. Examples of routing protocols are IGRP, OSPF, and RIP. routing table A list of the known paths and interfaces. routing The process of finding a path to a destination host. Routing is very complex in large networks because of the many potential intermediate destinations that a packet might traverse before reaching its destination host. shortest path first (SPF) algorithm A calculation performed on the database resulting in the SPF tree. static routing The process of manually defining and configuring the routes. topological database A collection of information gathered from LSAs. Check Your Understanding Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1. Which of the following best describes one function of Layer 3, the network layer, in the OSI model? A. It is responsible for reliable network communication between nodes. B. It is concerned with physical addressing and network topology. C. It determines which is the best path for traffic to take through the network. D. It manages data exchange between presentation layer entities. 2. What function allows routers to evaluate available routes to a destination and to establish the preferred handling of a packet? A. Data linkage B. Path determination C. SDLC interface protocol D. Frame Relay 1102.book Page 706 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 707 3. How does the network layer forward packets from the source to the destination? A. By using an IP routing table B. By using ARP responses C. By referring to a name server D. By referring to the bridge 4. What two parts of a network layer address do routers use to forward traffic through a network? A. Network address and host address B. Network address and MAC address C. Host address and MAC address D. MAC address and subnet mask 5. Which of the following best describes a routed protocol? A. Its address provides enough information to allow a packet to be forwarded from host to host. B. It provides information necessary to pass data packets up to the next-highest network layer. C. It allows routers to communicate with other routers to maintain and update address tables. D. It allows routers to bind MAC and IP addresses together. 6. Which of the following best describes a routing protocol? A. A protocol that accomplishes routing through the implementation of an algorithm B. A protocol that specifies how and when MAC and IP addresses are bound together C. A protocol that defines the format and use of fields within a data packet D. A protocol that allows a packet to be forwarded from host to host 7. What is one advantage of distance vector algorithms? A. They are not likely to count to infinity. B. You can implement them easily on very large networks. C. They are not prone to routing loops. D. They are computationally simple. 1102.book Page 707 Tuesday, May 20, 2003 2:53 PM 708 Chapter 15: Routing and Routing Protocols 8. Which of the following best describes a link-state algorithm? A. It recreates the exact topology of the entire internetwork. B. It requires numerous computations. C. It determines distance and direction to any link on the internetwork. D. It uses little network overhead and reduces overall traffic. 9. Why do routing loops occur? A. Slow convergence occurs after a modification to the internetwork. B. Split horizons artificially are created. C. Network segments fail catastrophically and take down other network segments in a cascade effect. D. Default routes never were established and initiated by the network administrator. 10. Which of the following best describes balanced hybrid routing? A. It determines best paths, but topology changes trigger routing table updates. B. It uses distance-vector routing to determine best paths between topology during high-traffic periods. C. It uses topology to determine best paths but does frequent routing table updates. D. It uses topology to determine best paths but uses distance vectors to circumvent inactive network links. 11. What is a network with only one path to a router called? A. Static network B. Dynamic network C. Entity network D. Stub network 12. Which best describes a default route? A. Urgent-data route manually entered by a network administrator B. Route used when part of the network fails C. Route used when the destination network is not listed explicitly in the routing table D. Preset shortest path 1102.book Page 708 Tuesday, May 20, 2003 2:53 PM . different systems. Figure 15 -25 shows an autonomous system. 11 02. book Page 703 Tuesday, May 20 , 20 03 2: 53 PM 704 Chapter 15 : Routing and Routing Protocols Figure 15 -25 Autonomous System Every. mode is to indicate IP 11 02. book Page 7 01 Tuesday, May 20 , 20 03 2: 53 PM 7 02 Chapter 15 : Routing and Routing Protocols network numbers. Dynamic routing uses broadcasts and multicasts to communicate. exterior routing protocol 11 02. book Page 7 02 Tuesday, May 20 , 20 03 2: 53 PM Autonomous Systems and IGP vs. EGP 703 Figure 15 -24 Routing Protocols Autonomous Systems and IGP vs. EGP An interior