Answers and Explanations

A1:	Answer: C. OSPF uses the highest IP address assigned to a loopback address, which is a logical interface created on a Cisco router. A is incorrect because Router ID is always the highest IP address. B is incorrect, as you have logical loopback interfaces on these routers. Logical interfaces override physical interfaces. If there were no loopback interfaces, the highest IP address on a physical interface would be the router ID. D is incorrect, as the router ID is always the highest IP address on a logical interface. E is incorrect, as the router ID is always the highest IP address on the either the logical or physical interface.
A2:	Answers: C and D. Routed protocols such as IP or IPX are used to help communicate data across network segments by using logical addressing, which identifies both the network and host that need to communicate. D is a correct answer, as routers use routing protocols such as RIP and OSPF to communicate changes in their routing tables. A is incorrect because routed protocols can cross routers. B is incorrect because routing protocols populate only routing tables; they cannot identify data.
A3:	Answer: A. A static route is the most believable route possible (except for being directly connected to a network by a router's interface). Routers use administrative distance (AD) to decide the best route. The lowest AD wins, and in this case, a static route has an AD of 1. B is incorrect, as RIP's AD is 120. C is incorrect, as OSPF's AD is 110. D is incorrect, as a router never "cancels" out routes if it hears from more than one source; that is the function of the AD.
A4:	Answer: D. The show IP route command shows what routes are available for the router. In this case, the information in the brackets is the administrative distance and metric (bandwidth and delay) of the EIGRP route. A is incorrect, as it is not the port number, but the administrative distance. B is incorrect, as EIGRP does not use hops as its metric. C is incorrect, as the first number is the AD of EIGRP, not port number.
A5:	Answer: C. This answer identifies all interfaces on a router with the 0.0.0.0 255.255.255.255 (All addresses) identifier. A is incorrect, as even though you are identifying all interfaces with Area 0, this command does not send out an LSA to the Area. B is incorrect, as the router id command configures a router with a specific router ID. D is incorrect, as this is an invalid command.
A6:	Answer: B. Open Shortest Path First (OSPF) is a link-state routing protocol that contains the entire topology of the network in a database. Because the routers know the entire network topology, they can choose the shortest path to a route's destination. A is incorrect, as RIP is a distance-vector routing protocol that contains only a routing table of routes learned by rumor through directly connected neighbors. C is incorrect, as IGRP is also a distance-vector routing protocol. D is incorrect, as BGP is a path-vector routing protocol used between autonomous systems on WANs and the Internet.
A7:	Answers: B, D, and F. You must specify the autonomous system (AS) number in the router igrp command. D is also correct, as you must specify the interface attached to the 192.168.2.0 network. F is correct, as you must specify the interface attached to the 192.168.3.0 network. A is incorrect because you do not give the AS as part of the command. C is incorrect because even though it might advertise for all network interfaces, it's not the approved methodology. E is incorrect because it's not a valid command.
A8:	Answer: D. With all distance-vector routing protocols, split-horizon states that it is not useful to send information back the way it came. A is incorrect, as routers themselves are the devices that split networks, not split-horizon. B is incorrect, as split-horizon is used to solve routing loop issues. C is incorrect because distance-vector routing protocols are notoriously slow in updating, as a result of their periodic nature. This can cause convergence issues.
A9:	Answer: D. The passive-interface command is used to control the advertisement of routing information. The command enables the suppression of routing updates over some interfaces while allowing updates to be exchanged normally over other interfaces. A is incorrect, as the actual command to turn off the interface is shutdown. B is incorrect, as this answer is the opposite of what the command does. C is incorrect, as keepalives keep up an Ethernet interface but do nothing for a serial interface.
A10:	Answer: A. The ip route 10.20.0.0 255.255.0.0 10.10.0.1 command establishes a static route in the routing table. B is incorrect, as static routes are specified using subnet masks rather than wildcard masks. Wildcard masks are typically used in access list and OSPF configurations. C and D are incorrect, as you must specify the IP syntax in front of the route command.
A11:	Answer: B. EIGRP is considered a hybrid routing protocol, combining the best of the distance-vector and link-state routing protocols. A, C, and D are incorrect, as there are no such classes of routing protocols.
A12:	Answer: A. The administrative distance (AD) of RIP is 120. B is incorrect, as IGRP has an AD of 100. C is incorrect, as OSPF has an AD of 110. D is incorrect, as EIGRP has an AD of 90.
A13:	Answer: C. The distance-vector mechanism to set the metric to the maximum value is called route poisoning. A is incorrect, even though route poisoning is part of the split-horizon family. Split-horizon, by definition, does not allow updates to travel back out the interface on which they arrived. B is incorrect, as hold-down timers are not activated by route poisoning. D is incorrect, as triggered updates occur when a change occurs on a network and require an update that is earlier than the default timer.
A14:	Answer: A. Distance-vector routing protocols send the entire routing table to directly connected neighbors. B is incorrect, as the routers send the tables only to their directly connected neighbors. If a router learns of another route from its neighbor, it then passes that information on to another neighbor. C is incorrect, as distance-vector routing protocols send the entire table, not just the changes. D is incorrect, as the routers send the entire table, and they send it only to directly connected neighbors.
A15:	Answer: B. The maximum hop count that RIP allows is 15. Answer C is incorrect because a hop count of 16 is the point that is considered unreachable by the RIP protocol. All other answers are incorrect because they either overshoot or underestimate the maximum hop count.
A16:	Answer: A. Split-horizon is a mechanism in distance-vector routing protocols that prevents routers from sending updates back the way they came. B is incorrect, as hold-down timers are used to prevent routing tables from responding too quickly to sudden changes. C is incorrect, as the Maximum Hop count refers to the maximum metric that RIP can use. D is incorrect, as route poisoning is the process of taking a route and making it the maximum distance, thus "poisoning" the route. E is incorrect, as "counting to infinity" is a problem if a routing update loop occurs.
A17:	Answer: B. The routing table information shown gives the administrative distance (AD) with the metric. A is incorrect, as the [/] shows the AD and metric. Cost is a metric used by OSPF. C is incorrect, as the AD is shown first, then the metric, not the other way around. D is incorrect, as the [AD/Metric] is not shown as cost and hop count.
A18:	Answer: D. The metric shown on this route is 15. If it is passed to a neighboring router, it increments by one, equaling 16, which is unreachable. A is incorrect, as this route could be passed to neighbors. B is incorrect, as this route could be passed to a neighboring router. C is incorrect, as a directly connected network could be advertised to a neighboring router. E is incorrect, as this route is also within the 15-hop limit.
A19:	Answer: B. EIGRP is called many names. Some call it a hybrid routing protocol (primarily Cisco); others call it an advanced distance-vector protocol. However, technically, it falls under the distance-vector category of protocols, which causes all the distance-vector loop prevention mechanisms to apply. One of these mechanisms is split-horizon, which prevents a router from sending an update back in the same direction from which it was received. By looking at the diagram, the Nevada router receives updates on its Serial 0/0 interface from both Washington and New Mexico. It does not send those updates back out, keeping the Washington and New Mexico routers from learning about each other. Answer A is incorrect because EIGRP works just fine on frame relay networks. Answer C is incorrect because there are no reserved EIGRP AS numbers. Answer D is incorrect because the no auto-summary command enables VLSM support for EIGRP.
A20:	Answer: A. Even though IGRP can use up to five different criteria (Bandwidth, Delay, Load, MTU, and Reliability), it uses only Bandwidth and Delay by default. B is incorrect, as IGRP has a maximum hop count of 255; however, it is not the metric it uses. C is incorrect, as ticks are used by RIP for IPX. D is incorrect, as IGRP uses not only Bandwidth, but also Delay as its default metrics.
A21:	Answer: A. RIP is considered a classful protocol. From a configuration standpoint, this means that networks must be entered into the configuration using their default classes (Class A, B, or C). Because 172.16.0.0 is a class B network, it must be entered as if it had a class B subnet mask rather than the subnet mask shown in the figure. In addition, you enter directly connected interfaces only into a routing process. This concept applies to any routing protocol. In the case of Ike, it is connected only to the 172.16.0.0 and 192.168.45.0 networks. Answers B and C are incorrect because they enter all the networks rather than the networks that are directly connected to Ike. Answer D is incorrect because it enters the 192.168.0.0 network in a class B style when this is a class C address.
A22:	Answer: B. A default route is a static route that contains a unique address and subnet mask combination of all zeros (0.0.0.0 0.0.0.0). A is incorrect, as it is a standard static route. C is incorrect, as it is a standard static route using an interface instead of a next-hop IP address. D is incorrect, as it is not even the correct syntax for a static route.
A23:	Answer: D. The show ip protocol command lists all routing protocols that are running on your router. A is incorrect because it shows you only the routed protocols, not the routing protocols, on your router. B is incorrect, because this is not a valid command. C is incorrect, as the show running-config command shows you what routing protocols are running, but it also gives you all the other configuration information currently operating in RAM. This is too much information, when all you wanted was to find out which routing protocol was running on the machine.
A24:	Answer: C. EIGRP is a distance-vector routing protocol that uses link-state attributes. A is incorrect, as OSPF is a link-state routing protocol, not a distance-vector routing protocol. B is incorrect, as IGRP is only a distance-vector routing protocol. D is incorrect, as RIPv1 is only a distance-vector routing protocol. E is incorrect, as BGP is a path-vector routing protocol that works between autonomous systems. F is incorrect, as IS-IS is a pure link-state routing protocol. G is incorrect, as RIPv2 is still a distance-vector routing protocol, although it does support VLSM.
A25:	Answers: D and F. You merely need to activate the routing protocols to turn on the routing loop-prevention mechanisms; they are activated by default. A and B are incorrect, as you actually use the no split-horizon command to turn it off and just the split-horizon command to turn it back on. Regardless, it is on by default. C and E are incorrect, as there are no such commands.
A26:	Answers: B, C, and D. Directly connected neighbors update each other when using the RIP routing protocol. A is incorrect, as Router A is not directly connected to Router C and thus does not send an update to Router C.
A27:	Answer: D. The running-config command shows that one router is running IGRP with an AS of 100, while the other is running IGRP with an AS of 110. These two routers do not share routing updates, as IGRP sends updates only to routers that belong to the same AS. A is incorrect, as the routers are connected by serial interfaces. B is incorrect, as the serial interfaces are all using PPP encapsulation. C is incorrect, as both the 172.16.0.0 and 10.0.0.0 networks are being advertised—just not to the other side!
A28:	Answer: B. The formula that OSPF uses to figure the cost of a path is 108/bandwidth (bps). In this case, 108/128000 = 781.25. A is incorrect, as that is the cost of a 100Mbps link. C is incorrect, as that is the cost of a 64K link. D and E are invalid costs.
A29:	Answer: D. The configuration shown in answer D is the most precise method to add interfaces to the OSPF routing process. A wildcard mask of 0.0.0.0 tells OSPF to run on the interface that has exactly the IP address that you have typed in before the wildcard mask. In this case, all the IP addresses were typed in exactly as defined on the interfaces with the wildcard mask of 0.0.0.0. This ensures OSPF does not run on any interfaces that are either not shown in the figure or that may be added in the future. Answer A accomplishes part of the objective because OSPF runs on any interface starting with 192.168.2 or 10 (because of the wildcard mask applied); however, in the future, if any interface is added to the router that has an IP address beginning with the number 10, it will automatically begin running OSPF. Answers B and C are incorrect because OSPF network statements require you to enter the wildcard mask rather than the subnet mask.
A30:	Answers: A, B, and D. The command creates a virtual loopback interface with a particular IP address using the 255.255.255.255 subnet mask. When an OSPF router sends a routing update, it includes the router ID to identify itself. In OSPF, the router ID is the highest IP address of a loopback interface. C is incorrect, as a router that is not powered up does not have any active interfaces. E is incorrect, as 255.255.255.255 is typically called a host mask. F is incorrect, as this command sets up a virtual interface, not a physical Ethernet interface.
A31:	Answer: B. One of the main features of classless protocols is to include the subnet mask information in their updates, whereas classful does not. This means that you can use only fixed-length subnet masks (FLSMs), not variable-length subnet masks (VLSMs) in a classful routing environment. A is incorrect, as classless protocols actually have more options. C is incorrect, as both classful and classless protocols can cross autonomous systems. D is incorrect, as RIPv1 and IGRP are actually classful protocols.
A32:	Answer: D. To show the specific routes of EIGRP, you must specify them with the show ip route EIGRP command. A is incorrect; although it shows EIGRP routes, it also shows any other routes learned or known by the router, and the question specifically asks for EIGRP routing table information. B is a nonexistent command. C is incorrect, as this command shows you all of the EIGRP neighbors for which the router exchanges information.
A33:	Answers: A and D. The show ip route and show ip protocols commands give you information on the routing protocols and what routes they have placed into the routing table. B is incorrect, as you cannot issue the show ip protocols command from Interface Configuration mode. C is incorrect, as you cannot use debug commands from the User EXEC mode. You must be in Privileged EXEC mode.
A34:	Answer: C. Monitoring in real time requires the use of a debug command. Show commands merely grab a snapshot in time, whereas debug commands are in real time. A is incorrect, as it is a show command. B is incorrect, as it is merely another show command. D is also incorrect, as it uses a show command.
A35:	Answers: D and E. Split-horizon ensures that updates are not sent back through the interface on which they arrived; this helps prevent loops. Hold-down timers prevent incorrect updates from propagating throughout the network. A is incorrect, as LSAs are used by link-state protocols to announce changes in the network. B is incorrect, as Spanning-Tree Protocol is used in switched environments, not routing protocols. C is incorrect because the SPF tree is used to help decide the best path to the destination.
A36:	Answers: A, C, and D. Link-state routing protocols have the entire network mapped in their topology database. From this database, it calculates the shortest path using Dijkstra's algorithm. They also send small update announcements called LSAs when there is a change in the topology. B is incorrect because link-state routing protocols exchange their routing information with their neighbors once and then send only changes. E is incorrect, as link-state routing protocols do not use split-horizon. F is incorrect, as link-state protocols exchange a periodic update, but only if no LSAs have been sent over a long period of time.
A37:	Answer: B. The use of VLSM in a routed network allows you to use different subnet masks throughout the network. You can also use route aggregation (also called route summarization), which shrinks the size of routing tables. A is incorrect, as you actually lose host addresses when you use subnetting. C is incorrect, as you can use different size subnet masks such as /30 and /12 and /26 masks in the same network. D is incorrect, as VLSM is not a loop-routing preventive mechanism.
A38:	Answer: B. IGRP has a default administrative distance (AD) of 100. A is incorrect, as the administrative distance of RIP is 120. C is incorrect, as OSPF has an AD of 110. D is incorrect, as EIGRP has an AD of 90.
A39:	Answers: A and C. You must turn on the routing protocol with the router rip command from Global Configuration mode. You must then specify interfaces to advertise; the network command accomplishes this. B is incorrect, as you do not need to specify an autonomous system for RIP, as you would IGRP. D is incorrect, as it is not a valid network command.
A40:	Answer: C. The correct syntax for a static route is ip route [destination network] [subnet mask] [interface or next hop ip address] [administrative distance(optional)]. Increasing the AD from the default of 1 to 90 allows the static route to be less believable than some routing protocols. This is known as a floating static route, which is used for backup paths. In this case, you are trying to reach the 192.18.40.0 network. You are telling the router to send packets for that network to 192.18.30.2, which is the next-hop IP address. You are also setting the AD to 90. A and B are incorrect, as the AD parameter does not go in front of the destination network parameter. D is incorrect, as you are trying to reach the 192.18.40.0 network, not the 192.18.30.0 network.
A41:	Answer: D. The show ip protocols command shows all currently configured routing protocols in use on your router. A is incorrect, as it shows you the routing table. B is incorrect, as it shows you the version of the IOS and the configuration register settings. C is incorrect, as it shows you all of the currently configured settings in RAM.
A42:	Answers: A, B, C, and E. A is correct, as RIPv1 is an Interior Gateway Protocol (IGP), meaning it exchanges routing information with routers in the same autonomous system (AS). B is correct, as RIPv2 is also an IGP. C is correct, as IGRP actually gets its name from being an IGP. E is correct, as OSPF is an industry standard IGP. D is incorrect, as Border Gateway Protocol is an External Gateway Protocol (EGP), which means it exchanges routing information between autonomous systems.
A43:	Answer: D. EIGRP is a proprietary Cisco routing protocol that does not function on other vendors' routers. OSPF, however, is an industry standard. A is incorrect, as EIGRP is actually a bit faster than OSPF in converging. B is incorrect, as EIGRP, although not technically a link-state protocol, uses link-state behavior. C is incorrect, as EIGRP's DUAL algorithm is equally capable of route calculation as OSPF's Dijkstra's algorithm.
A44:	Answer: A. Stub networks have only one way in and one way out. Static routes serve well in this environment. B is incorrect, as large corporations typically have hundreds of routers, which make static routes inappropriate. C is incorrect, as laptops versus desktops typically do not affect routing choices. D is incorrect, as static routes are typically used in high-security environments.
A45:	Answer: D. EIGRP has a default administrative distance of 90, which is the lowest of all of these protocols. A is incorrect, as RIP has an administrative distance of 120, which is the highest of all the protocols. B is incorrect, as IGRP has an administrative distance of 100. C is incorrect, as OSPF has an administrative distance of 110.
A46:	Answer: A. The ip route 172.16.0.0 255.255.0.0 172.31.0.1 command establishes a static route in the routing table. B is incorrect, as you do not have to specify that it is a static route with the word static. Instead, there should be an interface or next-hop router address parameter. C and D are incorrect, as you must specify the IP in front of the route command.
A47:	Answer: C. Poison reverse, or "reverse poisoning," overrides split-horizon and sends a route poison broadcast back out the interface on which the poisoned route came in. A is incorrect, as triggered updates override the periodic update behavior of distance-vector protocols. B is incorrect, as hold-down timers maintain stability of the routing table by waiting before making any changes. D is incorrect, as poison reverse overrides split-horizon.
A48:	Answers: A, B, and C. They are capable of being advertised with OSPF. D is incorrect, as the console interface is merely for local machine configuration and is not a valid network interface.
A49:	Answer: B. A router without active default mechanisms could theoretically increment its metric forever if a routing update loop occurs, thus the phrase "counting to infinity." All distance-vector routing protocols now have built-in mechanisms to stop this behavior. A is incorrect, as a router, if it does not know where to send a packet, either discards the packet or sends it to a "default route." C is incorrect, as a router does not run a calculation on a route it does not know. D is incorrect, as a router marks a route as "possibly down" when it receives a poisoned route update. It then activates a hold-down timer until it either receives an update that the route is back up, or it hears nothing and flushes the route from its table.
A50:	Answer: B. An OSPF router contains all known routes in its Topology table. It then runs the SPF algorithm for all the routes to decide the best path based on cost. A is incorrect, as the Neighbor table is merely the list of routers with which the OSPF router has a neighbor relationship. C is incorrect, as the Routing table merely contains the best routes calculated from the Topology table. D is incorrect, as the Content Addressable Memory Table is also known as the MAC address table. The CAM table is located on Catalyst switches.