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Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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14.1 Troubleshooting the routing process

In this section you are going to troubleshoot the demonstration network shown in Figure 1. You’ll look at the routing tables of the routers to see why ping packets cannot reach their destinations.

Watch the video, which is about 2 minutes long.

Troubleshooting the routing process

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Activity 1 Think about

10 minutes

1. In the video, when we pinged PC2 from PC1, the first ping timed out, whereas the remaining three succeeded. Write one or two sentences to explain what you think the reason is for this.

Answer

This happened because the ARP table in PC1 did not have an entry from PC2, so an ARP request was issued to find it. This process takes some time, which caused the first ping packet to time out.

2. An extract from the routing table of Router 1 from the demonstration network in the video is shown below.

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/0
L 10.10.10.1/32 is directly connected, GigabitEthernet0/0
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, GigabitEthernet0/1
L 192.168.1.1/32 is directly connected, GigabitEthernet0/1
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.2.0/24 is directly connected, GigabitEthernet0/2
L 192.168.2.1/32 is directly connected, GigabitEthernet0/2

Why can’t a ping from PC1 to PC3 get beyond Router 1? Choose the best answer from the options below.

(Hint: think about how we have set up our network so far.)

a. 

An interface has had its IP address wrongly configured.


b. 

One of the routers has an interface down.


c. 

There is no route in Router 1’s routing table for the destination network.


d. 

The destination PC is switched off.


The correct answer is c.

c. 

Yes. In Router 1’s routing table, there is no known route from Router 1 to Router 2 for packets addressed to anything in the 172.16.1.0/24 network.