IP Routing Questions and Answers

Answer: B

Correct answer: debug ip rip

debug ip rip shows the updates as they are sent and received, with the networks and metrics in each. show ip route displays the result of those updates rather than the updates themselves, and there is no debug ip route command. As always with debug, it should be enabled briefly on a production router.

Answer: B

Correct answer: 3 and 5

By default IGRP uses bandwidth and delay, combining them into a composite metric. Reliability, load and MTU can be included by setting the K values, but they are switched off by default, and MTU is carried in the update without being used in the calculation. Hop count is RIP’s metric, and STP is a switching protocol with no part in routing.

Answer: D

There are actually three different ways to configure the same default route, but only two are shown in the answer. First, you can set a default route with the 0.0.0.0 0.0.0.0 mask and then specify the next hop, as in option A. Or you can use 0.0.0.0 0.0.0.0 and use the exit interface instead of the next hop. Finally, you can use option D with the ip default-network command.

Answer: B

When a routing update is received by a router, the router first checks the administrative distance (AD) and always chooses the route with the lowest AD. However, if two routes are received and they both have the same AD, then the router will choose the one route with the lowest metrics, or in RIP's case, hop count.

Answer: A

Correct answer: Information about a route should not be sent back in the direction from which the original update came.

Split horizon is a loop prevention rule: a router does not advertise a route back out of the interface it learned it on, since the neighbour there already has a better path and would otherwise be told about its own route at a worse metric. It is one of several such mechanisms in RIP, alongside route poisoning, hold-down timers and triggered updates.

Answer: B

Correct answer: IGRP has a lower administrative distance.

When two protocols offer a route to the same network, the router compares administrative distance first and never reaches the metrics - which are not comparable between protocols in any case, hop count meaning nothing beside a composite of bandwidth and delay. IGRP’s distance is 100 against RIP’s 120, so the IGRP route is installed and the RIP one is held in reserve.

Answer: C

Correct answer: It has the same timers as RIPv1.

RIPv2 keeps the same 30 second update, 180 second invalid and 240 second flush timers, so it converges no faster than RIPv1. What it adds is the subnet mask in its updates - making it classless - along with multicast rather than broadcast updates and optional authentication. Its administrative distance is 120, the same as RIPv1, and it is configured almost identically.

Answer: A

Although answer D almost seems right, it is not; the mask is the mask used on the remote network, not the source network. Since there is no number at the end of the static route, it is using the default administrative distance of 1.

Answer: C

Correct answer: Router(config-router)# passive-interface s0

Two things must be right: the command belongs under the routing process rather than on the interface, and the interface is named as its argument. The effect is one-way - the interface stops sending updates while continuing to receive and process them - which suits a link to a stub network or to a device that should not hear the routing protocol.

Answer: C

Static routes have an administrative distance of 1 by default. Unless you change this, a static route will always be used over any other found route. IGRP has an administrative distance of 100, and RIP has an administrative distance of 120, by default.