IPv4 Routing Protocol Concepts

Routers and Layer 3 switches add IP routes to their routing tables using three methods: connected routes, static routes, and routes learned by using dynamic routing protocols. The routing process forwards IP packets, but if a router does not have any routes in its IP routing table that match a packet’s destination address, the router discards the packet. Routers need routing protocols so that the routers can learn all the possible routes and add them to the routing table, so that the routing process can forward (route) routable protocols such as IP.

IPv4 supports several different routing protocols, some of which are primarily used inside one company, while one is meant primarily for use between companies to create the Internet. This chapter introduces the concepts behind Interior Gateway Protocols (IGPs), typically used inside one company. In particular, this chapter discusses the theory behind types of routing protocols, including distance vector and link-state logic. The chapter also introduces the Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF) routing protocol.

“Do I Know This Already?” Quiz

Use the “Do I Know This Already?” quiz to help decide whether you might want to skim this chapter, or a major section, moving more quickly to the “Exam Preparation Tasks” section near the end of the chapter. You can find the answers at the bottom of the page following the quiz. For thorough explanations, see Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes.”

Table 20-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

1. Which of the following distance vector features prevents routing loops by causing the routing protocol to advertise only a subset of known routes, as opposed to the full routing table, under normal stable conditions?

   1. Route poisoning
   2. Dijkstra SPF
   3. Hello
   4. Split horizon

2. Which of the following distance vector features prevents routing loops by advertising an infinite metric route when a route fails?

   1. Dijkstra SPF
   2. Hello
   3. Split horizon
   4. Route poisoning

3. Which of the following is true about both RIPv1 and RIPv2? (Choose two answers.)

   1. Uses a hop-count metric
   2. Sends update messages to multicast address 224.0.0.9
   3. Supports authentication
   4. Uses split horizon

4. Router R1 uses RIPv1, and learns one possible route to reach subnet 10.1.1.0/24. That route would have a metric of 15 from R1’s perspective. Which of the following is true?

   1. R1 cannot use the route, because metric 15 is considered to be infinity.
   2. R1 will add the route to its routing table.
   3. The cumulative bandwidth between R1 and subnet 10.1.1.0/24 is 15 Mbps.
   4. The slowest bandwidth of the links between R1 and subnet 10.1.1.0/24 is 15 Kbps.

5. Routers A and B use EIGRP. How does router A watch for the status of router B so that router A can react if router B fails?

   1. By using EIGRP hello messages, with A needing to receive periodic hello messages to believe B is still working
   2. By using EIGRP update messages, with A needing to receive periodic update messages to believe B is still working
   3. Using a periodic ping of B’s IP address based on the EIGRP neighbor timer
   4. None of the other answers are correct.

6. Which of the following affect the calculation of EIGRP metrics when all possible default values are used? (Choose two answers.)

   1. Bandwidth
   2. Delay
   3. Load
   4. Reliability
   5. Hop count

7. Which of the following routing protocols are considered to use link-state logic?

   1. RIPv1
   2. RIPv2
   3. EIGRP
   4. OSPF

8. Which of the following is true about how a router using a link-state routing protocol chooses the best route to reach a subnet?

   1. The router finds the best route in the link-state database.
   2. The router calculates the best route by running the SPF algorithm against the information in the link-state database.
   3. The router compares the metrics listed for that subnet in the updates received from each neighbor and picks the best (lowest) metric route.
   4. The router uses the path that has the lowest hop count.