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IP Routing Questions and Answers

11.

In hierarchical routing with 4800 routers, what region and cluster sizes should be chosen to minimize the size of the routing table for a threelayer hierarchy ?

Exam Year(s)

UGC NET - 2013,

Answer: D

For three level hierarchical routing the routing table has one entry for each cluster, one for each region of the local cluster and one for each router of the local region, so its size is the sum of the three numbers, while their product must equal the 4800 routers. Option (A) 10 x 24 x 20 = 4800 gives a table of 54, option (B) 12 x 20 x 20 = 4800 gives 52, option (C) 16 x 12 x 25 = 4800 gives 53, and option (D) 15 x 16 x 20 = 4800 gives 51. All four have the right product, and the smallest sum is 51 for 15 clusters, 16 regions and 20 routers, option (D). The sum is smallest when the three factors are as nearly equal as possible, near the cube root of 4800 which is about 16.9.

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12.

The count-to-infinity problem is associated with

Exam Year(s)

UGC NET - 2013,

Answer: C

The count-to-infinity problem is associated with the distance vector routing algorithm. When a link fails, routers keep exchanging increasing distance estimates through each other and the metric climbs slowly towards infinity before the route is dropped.

Link state routing floods the full topology, so every router computes the paths itself and does not suffer from this problem. Flooding and hierarchical routing are not distance-vector exchanges. Option C is correct. The usual remedies are split horizon, poison reverse and defining a small value as infinity, such as 16 in RIP.

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13.

What is route poisoning?

Answer: D

Another way to avoid problems caused by inconsistent updates and to stop network loops is route poisoning. When a network goes down, the distance-vector routing protocol initiates route poisoning by advertising the network with a metric of 16, or unreachable (sometimes referred to as infinite).

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14.

What does RIPv2 use to prevent routing loops?

1.CIDR

2.Split horizon

3.Authentication

4.Classless masking

5.Holddown timers

Answer: C

RIPv2 uses the same timers and loop-avoidance schemes as RIPv1. Split horizon is used to stop an update from being sent out the same interface it was received on. Holddown timers allow time for a network to become stable in the case of a flapping link.

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15.

Which two of the following are true regarding the distance-vector and link-state routing protocols?

1.Link state sends its complete routing table out all active interfaces on periodic time intervals.

2.Distance vector sends its complete routing table out all active interfaces on periodic time intervals.

3.Link state sends updates containing the state of its own links to all routers in the internetwork.

4.Distance vector sends updates containing the state of its own links to all routers in the internetwork.

Answer: C

The distance-vector routing protocol sends its complete routing table out all active interfaces at periodic time intervals. Link-state routing protocols send updates containing the state of its own links to all routers in the internetwork.

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16.

You have the following routing table. Which of the following networks will not be placed in the neighbor routing table?

R    192.168.30.0/24 [120/1] via 192.168.40.1, 00:00:12, Serial0
C    192.168.40.0/24 is directly connected, Serial0
     172.16.0.0/24 is subnetted, 1 subnets
C    172.16.30.0 is directly connected, Loopback0
R    192.168.20.0/24 [120/1] via 192.168.40.1, 00:00:12, Serial0
R    10.0.0.0/8 [120/15] via 192.168.40.1, 00:00:07, Serial0
C    192.168.50.0/24 is directly connected, Ethernet0

Answer: C

Correct answer: 10.0.0.0

The 10.0.0.0/8 route already has a metric of 15, and RIP adds one hop when advertising it onward, giving 16 - which RIP defines as infinity, meaning unreachable. The neighbour therefore discards it. The other entries have metrics of 0 or 1 and will be advertised and installed normally.

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17.

Which statement is true regarding classless routing protocols?

1.The use of discontiguous networks is not allowed.

2.The use of variable length subnet masks is permitted.

3.RIPv1 is a classless routing protocol.

4.IGRP supports classless routing within the same autonomous system.

5.RIPv2 supports classless routing.

Answer: C

Correct answer: 2 and 5

A classless protocol carries the subnet mask in its updates, which is what permits variable length subnet masks and discontiguous networks - so statement 2 is correct and statement 1 states the opposite. RIPv2 is classless, so statement 5 is correct. RIPv1 and IGRP are both classful and send no mask, which disposes of statements 3 and 4.

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18.

The Corporate router receives an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3. Looking at the output from the Corporate router, what will the router do with this packet?

Corp#sh ip route
[output cut]
R    192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0
R    192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
R    192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
C    192.168.20.0 is directly connected, Serial0/0
C    192.168.214.0 is directly connected, FastEthernet0/0

Answer: A

Since the routing table shows no route to the 192.168.22.0 network, the router will discard the packet and send an ICMP destination unreachable message out interface FastEthernet 0/0, which is the source LAN where the packet originated from.

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19.

Which command displays RIP routing updates?

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.

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20.

IGRP uses which of the following as default parameters for finding the best path to a remote network?

1.Hop count

2.MTU

3.Cumulative interface delay

4.STP

5.Path bandwidth value

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.

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