14.1 Troubleshooting the routing process

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 192.168.2.2 – 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 10.10.10.2 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.