IP Routing Questions and Answers

Q: A network administrator views the output from the show ip route command. A network that is advertised by both RIP and IGRP appears in the routing table flagged as an IGRP route. Why is the RIP route to this network not used in the routing table?

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.

Q: Which of the following is true regarding RIPv2?

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.

Q: What command is used to stop RIP routing updates from exiting out an interface but still allow the interface to receive RIP route updates?

Correct answer: Router(config-router)# passive-interface s0Two 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.

Q: You type debug ip rip on your router console and see that 172.16.10.0 is being advertised to you with a metric of 16. What does this mean?

Correct answer: The route is inaccessible.RIP counts to infinity at 16, so a metric of 16 is not a distance but a poison: it is how a router announces that a route it once advertised is gone. Since the maximum usable metric is 15 hops, any RIP network wider than that cannot be reached at all - which is one of the protocol’s principal limitations.

21.

Network 206.143.5.0 was assigned to the Acme Company to connect to its ISP. The administrator of Acme would like to configure one router with the commands to access the Internet. Which commands could be configured on the Gateway router to allow Internet access to the entire network?

1.Gateway(config)# ip route 0.0.0.0 0.0.0.0 206.143.5.2

2.Gateway(config)# router rip

3.Gateway(config-router)# network 206.143.5.0

4.Gateway(config-router)# network 206.143.5.0 default

A.1 only
B.3 only
C.1, 2 and 4
D.1 and 4

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.

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22.

Two connected routers are configured with RIP routing. What will be the result when a router receives a routing update that contains a higher-cost path to a network already in its routing table?

A.The updated information will be added to the existing routing table.
B.The update will be ignored and no further action will occur.
C.The updated information will replace the existing routing table entry.
D.The existing routing table entry will be deleted from the routing table and all routers will exchange routing updates to reach convergence.

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.

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23.

What is split horizon?

A.Information about a route should not be sent back in the direction from which the original update came.
B.It splits the traffic when you have a large bus (horizon) physical network.
C.It holds the regular updates from broadcasting to a downed link.
D.It prevents regular update messages from reinstating a route that has gone down.

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.

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24.

A network administrator views the output from the show ip route command. A network that is advertised by both RIP and IGRP appears in the routing table flagged as an IGRP route. Why is the RIP route to this network not used in the routing table?

A.IGRP has a faster update timer.
B.IGRP has a lower administrative distance.
C.RIP has a higher metric value for that route.
D.The IGRP route has fewer hops.

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.

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25.

Which of the following is true regarding RIPv2?

A.It has a lower administrative distance than RIPv1.
B.It converges faster than RIPv1.
C.It has the same timers as RIPv1.
D.It is harder to configure than RIPv1.

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.

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26.

Which of the following statements are true regarding the command ip route 172.16.4.0 255.255.255.0 192.168.4.2?

1.The command is used to establish a static route.

2.The default administrative distance is used.

3.The command is used to configure the default route.

4.The subnet mask for the source address is 255.255.255.0.

A.1 and 2
B.2 and 4
C.3 and 4
D.All of the above

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.

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27.

What command is used to stop RIP routing updates from exiting out an interface but still allow the interface to receive RIP route updates?

A.Router(config-if)# no routing
B.Router(config-if)# passive-interface
C.Router(config-router)# passive-interface s0
D.Router(config-router)# no routing updates

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.

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28.

If your routing table has a static, a RIP, and an IGRP route to the same network, which route will be used to route packets by default?

A.Any available route
B.RIP route
C.Static route
D.IGRP route
E.They will all load-balance.

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.

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29.

You type debug ip rip on your router console and see that 172.16.10.0 is being advertised to you with a metric of 16. What does this mean?

A.The route is 16 hops away.
B.The route has a delay of 16 microseconds.
C.The route is inaccessible.
D.The route is queued at 16 messages a second.

Answer: C

Correct answer: The route is inaccessible.

RIP counts to infinity at 16, so a metric of 16 is not a distance but a poison: it is how a router announces that a route it once advertised is gone. Since the maximum usable metric is 15 hops, any RIP network wider than that cannot be reached at all - which is one of the protocol’s principal limitations.

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