518Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
17.Assume that a router is configured to allow only one route in the routing table to each destination network. If more than one route to a particular subnet is learned, and if each route has the same metric value, which route is placed in the routing table if the routing protocol uses distance vector logic?
Answer: In this scenario, the first route learned is placed in the table. If that route is removed later, the next routing update received after the original route has been removed is added to the routing table.
18.Describe the purpose and meaning of route poisoning.
Answer: Route poisoning is the distance vector routing protocol feature in which a newly bad route is advertised with an infinite metric. Routers receiving this routing information then can mark the route as a bad route immediately. The purpose is to prevent routing loops.
19.Describe the meaning and purpose of triggered updates.
Answer: A triggered update is the routing protocol feature in which an update is sent immediately when new routing information is learned rather than waiting on a timer to complete before sending another routing update.
20.What term describes the underlying logic behind the OSPF routing protocol?
Answer: Link state
21.Router1 has a serial interface S0 connected via a point-to-point link to Router2. Router2 has an Ethernet interface address of 20.1.21.1, mask 255.255.252.0. Write down the single variation of the ip route command for which you now have enough information to configure a complete, syntactically correct command.
Answer: You must use the ip route 20.1.20.0 255.255.255.252 serial0 command instead of ip route 20.1.20.0 255.255.252.0 next-hop. Both work, but because you do not know Router2’s IP address on the serial link, you do not have enough information to configure the command in the style that refers to the next-hop IP address.
Chapter 6
“Do I Know This Already?” Quiz
1.Which of the following affects the calculation of OSPF routes when all possible default values are used?
Answer: A. OSPF calculates metrics based on the cost associated with each interface. OSPF, by default, calculates interface cost based on the bandwidth setting.
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2.Which of the following affects the calculation of EIGRP metrics when all possible default values are used?
Answer: A, B. Like IGRP, EIGRP by default uses bandwidth and delay when calculating its metrics.
3.OSPF runs an algorithm to calculate the currently-best route. Which of the following terms refer to that algorithm?
Answer: A, D. OSPF uses the SPF algorithm, conceived by a mathematician named Dijkstra.
4.EIGRP uses an algorithm to find routes when no backup route exists. Which of the following terms refers to that algorithm?
Answer: B. A feasible successor route is considered a backup route. If one does not exist, EIGRP uses the DUAL algorithm to find any possible routes and to ensure that no loops exist when using that route.
5.How do OSPF and EIGRP notice when a neighboring router fails?
Answer: D. OSPF and EIGRP both use Hello messages, but they use Hello messages unique to that routing protocol.
6.Which of the following network commands, following the command router ospf 1, tell this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1?
Answer: B, D. Answer D works because the wildcard mask matches all IP addresses that start with 10 and end with 1. Answer B also works, because it matches all interfaces in Class A network 10.0.0.0.
7.Which of the following network commands, following the command router ospf 1, tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1?
Answer: A
8.Which of the following commands list the OSPF neighbors off interface serial 0/0?
Answer: A, B, E
9.In the show ip route command, what code designation implies that a route was learned with EIGRP?
Answer: I
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4.Create a configuration for EIGRP on a router with these interfaces and addresses: e0 using 200.1.1.1, e1 using 128.1.3.2, s0 using 192.0.1.1, and s1 using 223.254.254.1.
router eigrp 1 network 200.1.1.0 network 128.1.0.0 network 192.0.1.0 network 223.254.254.0
Answer: Because four different networks are used, four network commands are required. If you noticed that this question does not specify the AS number (1 in this example) but configures one, you get full credit. A few of these network numbers are used in examples; memorize the range of valid Class A, B, and C network numbers.
5.From a router’s user mode, without using debugs or privileged mode, how can you determine what routers are sending you EIGRP routing updates?
Answer: The show ip protocol command output lists the routing sources—the IP addresses of routers sending updates to this router. Knowing how to determine a fact without looking at the configuration will better prepare you for the exam. Also, the show ip route command lists next-hop router IP addresses. The next-hop routers listed identify the routers that are sending routing updates. The show ip eigrp neighbor and show ip ospf interface commands also list neighbors that by definition send routing updates.
6.If the command router eigrp 1, followed by network 10.0.0.0, with no other network commands, is configured in a router that has an Ethernet0 interface with IP address 168.10.1.1, does EIGRP send updates out Ethernet0?
Answer: No. There must be a network statement for network 168.10.0.0 before EIGRP advertises out that interface. The network command simply selects the connected interfaces on which to send and receive updates.
7.If the command router ospf 1, followed by network 10.0.0.0 0.255.255.255 area 0, with no other network commands, is configured in a router that has an Ethernet0 interface with IP address 10.10.1.1, does OSPF send updates out Ethernet0?
Answer: Not necessarily. OSPF must discover other OSPF neighbors on the interface before it advertises routing information (LSAs.)
8.If the commands router eigrp 1 and network 10.0.0.0 are configured in a router that has an Ethernet0 interface with IP address 168.10.1.1, mask 255.255.255.0, does this router have a route to 168.10.1.0?
Answer: Yes. The route is in the routing table because it is a directly connected subnet, not because of any action by EIGRP.
522Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
9.Which command lists all IP routes learned via OSPF?
Answer: The show ip route ospf command lists only OSPF-learned routes.
10.Compare and contrast the type of information exchanged in routing updates sent by distance vector routing protocols versus link-state protocols.
Answer: Distance vector protocols advertise subnets and their associated metric values. Link-state protocols advertise information about routers and subnets, or links, in the network, along with metric information for the links. Link-state protocols describe the full topology in the network. As a result, link-state routing information is much more detailed than distance vector protocols.
11.Define balanced hybrid, and give an example of a balanced hybrid protocol.
Answer: Cisco uses the term balanced hybrid to describe a class of routing protocols that have some distance vector characteristics and some link-state characteristics. Currently, only EIGRP falls into this category.
12.Describe how balanced hybrid protocols differ from distance vector protocols in terms of how a router notices that a neighboring router has failed.
Answer: Distance vector routing protocols rely on regular full routing updates from each neighboring router. If a router fails to receive those updates for a period of time, the router that fails to receive the updates assumes that the other router has failed. Balanced hybrid protocols do not send full updates regularly; instead, they send periodic Hello messages. If Hello messages are not received for some timeout period, the router that is no longer receiving the updates assumes that the other router has failed.
13.List the distance vector loop-avoidance features used by OSPF, such as split horizon.
Answer: OSPF, as a link-state routing protocol, does not need to use any of the distance vector loop-avoidance features. Loop avoidance is effectively built into the routing protocol.
14.List two OSPF features that help decrease the size of the OSPF topology database.
Answer: If you use multiple OSPF areas, the size of the database in routers that are not ABRs decreases. Also, by using some stub area type, you can reduce the size of the topology database even further.
15.Assume that you must choose between OSPF and EIGRP for a routing protocol in a new network you are building. List and explain the most compelling reason to choose OSPF and the most compelling reason to choose EIGRP.