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

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Discovering computer networks: hands on in the Open Networking Lab

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

Download this video clip.Video player: 75_troubleshooting_the_routing_process.mp4
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In an earlier session we looked at the job a router does – which is to route packets of data around our networks, so that the packet will get to the desired destination. We looked at how routers make their routing decisions by looking up the destination address in their routing tables and then sending the packet on its way via the appropriate port.

In this session we are going to look at how routers learn about distant networks that they are not directly connected to. We are going to start with the following network, which we have cabled up and configured all the IP addresses on.

We have setup a ping from PC1 to – which is the address of PC2. We can see here that PC1 can ping PC2 successfully. You should be able to explain already why the first of the pings has timed out.

You would expect this ping to work as both networks are local to Router1 and these networks will be entered into the routing table of the local router automatically when the IP addresses are entered and the interfaces come up.

We are going to ping the gigabit 0/0 interface of Router1 from PC1. This again should be successful as it is local to the same router.

Next we are going to ping the IP address at the other end of the cable linking the routers. So we execute ping from PC1. As we can see, this has failed – even after allowances for any ARP lookups.

Just for good measure we will now ping PC3 from PC1. We would expect this to fail as well, as we couldn’t even reach Router2 from the previous ping.

To try and work out what is wrong, we will now look at the routing tables of both routers. We can see that in Router1 we have entries for the directly connected networks and in Router 2 there are the directly connected networks here as well. We need to think about why PC1 cannot ping PC3.

End transcript
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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.


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 is variably subnetted, 2 subnets, 2 masks C is directly connected, GigabitEthernet0/0 L is directly connected, GigabitEthernet0/0 is variably subnetted, 2 subnets, 2 masks C is directly connected, GigabitEthernet0/1 L is directly connected, GigabitEthernet0/1 is variably subnetted, 2 subnets, 2 masks C is directly connected, GigabitEthernet0/2 L 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.)


An interface has had its IP address wrongly configured.


One of the routers has an interface down.


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


The destination PC is switched off.

The correct answer is 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 network.


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