First steps: topology, addressing and the first ping

1If you have never done this before

A computer network is made up of three kinds of thing. That is all. The whole rest of the course adds detail on top of this sketch.

Two ways of working

You have a choice, and both variants are accepted for submission:

• The workbench in this page. Nothing to install. The equipment is simulated here, in the browser, with the same commands as on real hardware. Start straight from section 4.
• Cisco Packet Tracer, installed on your own computer. It is closer to reality, it has the visual simulation mode, and you will need it in the following laboratories. Installing it requires a free Cisco Networking Academy account.

2Equipment needed

The complete list for this exercise. In Packet Tracer the equipment is taken from the bottom bar: choose the category on the left first, then the model on the right, then click in the workspace.

In addition, for yourselves: a notebook or a file in which to write the addressing plan and the answers, and about two hours.

Which cable, between which devices

The rule has a single idea: two devices of the same kind need a cable that swaps the wires; two devices of different kinds do not.

3Five words, once and for all

The rest of the exercise uses these five notions at every step. Do not move on until they sound familiar.

4The first network: two computers

Before the complete topology, a network as small as it can be: two computers and a switch. Here you learn the mechanics - beyond this it is all repetition.

In the workbench below, click PC0, fill in the address 192.168.10.1 and the mask 255.255.255.0 in the right-hand panel, then press apply. Do the same for PC1, with the address 192.168.10.2. Go back to PC0 and type in the terminal:

ping 192.168.10.2

5The complete topology

The addressing plan

This is the table you fill in before touching any equipment. In practice, an addressing plan written in advance prevents more problems than any troubleshooting command.

6The command line, in three minutes

Routers and switches have neither a screen nor a mouse. They are configured by typing commands into a console, in the Cisco IOS operating system. The console looks intimidating for two minutes, and then becomes the fastest tool you will ever use.

How to read the prompt

The text before the cursor always tells you which device you are on and in which mode. It is never typed by hand: the device displays it.

You come back down with exit (one level) or end (straight to privileged mode).

7Working steps

The workbench below holds the complete topology of figure 1, already cabled. Follow the steps in order; the list of objectives on the right ticks itself off as you progress.

1. Build the topology Packet Tracer only

   Place the two switches, the router, the three hosts and the server. Join them exactly as in figure 1, with Copper Straight-Through cable. Save the file as lab01_name.pkt and get into the habit of saving often.

   Watch the colour of the cable ends: green means the link is up, red means the link has not come up, and amber, between two switches, means the port is going through the STP states and will turn green in about 30 seconds.

   In the workbench in this page the topology is already cabled; go on to step 2.

2. Configure the four hosts

   In Packet Tracer: click PC0 → the Desktop tab → IP Configuration. In the workbench in this page: click PC0 → the Desktop → IP Configuration panel on the right. In both cases fill in the three fields from the addressing plan and repeat for PC1, PC2 and Server0.

   Check, from Desktop → Command Prompt (or directly in the workbench terminal):

   on PC0copy

   ipconfig
   ping 192.168.1.11
   

   The second ping must work already: PC0 and PC1 are in the same network, and the switch joins them directly - the router plays no part here. The result looks like this:

   what should appear

   Pinging 192.168.1.11 with 32 bytes of data:
   
   Request timed out.
   Reply from 192.168.1.11: bytes=32 time<1ms TTL=128
   Reply from 192.168.1.11: bytes=32 time<1ms TTL=128
   Reply from 192.168.1.11: bytes=32 time<1ms TTL=128
   
   Ping statistics for 192.168.1.11:
       Packets: Sent = 4, Received = 3, Lost = 1 (25% loss),
   

   The first packet is almost always lost. This is not a fault - it is ARP, and in step 8 you see exactly why.

3. Configure the router

   In Packet Tracer: click R1 → the CLI tab → press Enter. If it asks "Would you like to enter the initial configuration dialog?", answer no. In the workbench in this page: click R1 and type straight into the terminal.

   Type the commands below line by line, pressing Enter after each. The prompt changes along the way - compare it with what you see at the left of each line:

   the basic configuration of R1copy

   Router> enable
   Router# configure terminal
   Router(config)# hostname R1
   R1(config)# no ip domain-lookup
   
   R1(config)# interface gigabitEthernet 0/0/0
   R1(config-if)# description Link towards network A
   R1(config-if)# ip address 192.168.1.1 255.255.255.0
   R1(config-if)# no shutdown
   R1(config-if)# exit
   
   R1(config)# interface gigabitEthernet 0/0/1
   R1(config-if)# description Link towards network B
   R1(config-if)# ip address 192.168.2.1 255.255.255.0
   R1(config-if)# no shutdown
   R1(config-if)# end
   
   R1# copy running-config startup-config
   

   What each group of commands does:

   Command	Its effect
   enable + configure terminal	opens the right to change the device
   hostname R1	renames the router; from now on the prompt reads R1#
   interface …	enters the configuration of a single physical interface
   ip address …	gives the interface the gateway address of its network
   no shutdown	brings the interface up
   copy running-config startup-config	saves, so that the work survives a restart

   The two commands everybody forgets no shutdown: the interfaces of a router are shut down by default, unlike those of a switch. Without this command, the interface has an address but is dead.
   copy running-config startup-config: without it, all the work vanishes when the device restarts. running-config is the configuration in working memory; startup-config is the one read at boot.
   no ip domain-lookup is not compulsory, but it spares you the 30-second wait that occurs whenever you mistype a command and IOS tries to resolve it through DNS.
4. Check the state of the interfaces
   on R1copy

   R1# show ip interface brief
   

   You must see exactly this - both configured interfaces with up in both columns:

   what should appear

   Interface              IP-Address      OK? Method Status                Protocol
   GigabitEthernet0/0/0   192.168.1.1     YES manual up                    up
   GigabitEthernet0/0/1   192.168.2.1     YES manual up                    up
   GigabitEthernet0/0/2   unassigned      YES unset  administratively down down
   

   The first status column refers to the physical layer (is there a signal on the wire?), the second to the line protocol (does the link actually work?). If you do not see this, the table below tells you what is missing:

   What you see	What it means	What you do
   up / up	all is well	carry on
   administratively down	no shutdown is missing	re-enter the interface and issue no shutdown
   down / down	no signal	missing cable, wrong end, or the device opposite is switched off
   up / down	there is a signal, but no protocol	usually an encapsulation mismatch; rare in this exercise
   unassigned	the interface has no address	you configured an interface other than the one that is cabled

5. Look at what the router knows about the world
   on R1copy

   R1# show ip route
   

   Exactly two routes marked C (directly connected networks) must appear, along with its own addresses marked L (local). The router knows only what is attached to it, and nobody has told it anything else. From laboratory 5 onwards we begin to tell it.

6. Test connectivity, in increasing order
   on PC0copy

   ping 192.168.1.1
   ping 192.168.2.1
   ping 192.168.2.10
   tracert 192.168.2.10
   

   The four commands rise in difficulty: one's own gateway, the far interface of the router, the server in the other network, then the complete path. If one of them fails, the problem is exactly between it and the previous one - it is the quickest way of locating a fault and you will use it all semester.

   tracert must show two hops: first 192.168.1.1 (the router), then 192.168.2.10 (the server).

7. Look at the ARP table of the host
   on PC0copy

   arp -a
   

   You will see entries for 192.168.1.11 (PC1, in the same network) and for 192.168.1.1 (the gateway). You will see nothing for 192.168.2.10, although you have just pinged it successfully.

   Why the server does not appear in the ARP table Because PC0 never spoke to it directly. It sent the frames to the MAC address of the gateway, and from there the router took over. The destination IP address remained that of the server; the destination MAC address was, the whole time, that of R1. This is the rule of the two addresses, which you verify experimentally in the following section.
8. Look at the MAC table of the switch
   on SW1copy

   Switch> enable
   Switch# show mac address-table
   Switch# clear mac address-table dynamic
   Switch# show mac address-table
   

   The first display shows a populated table, although nobody configured it: it filled itself from the traffic that passed through. After clear it is empty. Issue a ping from PC0 and display it again - it fills up in less than a second.

8What a packet carries, segment by segment

This is the part for which the laboratory is worth doing. A ping from PC0 to Server0 crosses four links. On each of them, the packet carries a pair of IP addresses and a pair of MAC addresses. The question is: which of them change on the way?

In the workbench in this page

Issue the command ping 192.168.2.10 on PC0 and look at the Path of the last packet table, which appears under the terminal. It has one row for each link crossed.

In Packet Tracer

1. Switch from Realtime to Simulation, in the bottom right corner.
2. Press Edit Filters, deselect everything and leave only ICMP and ARP ticked.
3. From PC0 issue ping 192.168.2.10, then press Capture / Forward step by step.
4. Click the envelope and then the Inbound PDU Details tab to read the addresses.

The order of events is always the same:

1. The first packet is an ARP request, in broadcast: PC0 is looking for the MAC address of the gateway. Notice that the switch sends it out of every port.
2. R1 answers with an ARP reply, this time unicast, only towards PC0.
3. Only now does the ICMP echo request leave - the real ping.
4. Follow the same packet after it leaves R1 on G0/0/1. The IP addresses are the same. The MAC addresses are completely different.
5. Follow the reply all the way back to PC0.

Fill in the table below for the ICMP echo request packet, on each segment. It is one of the deliverables of the exercise:

9Three faults to diagnose

Introduce each of the faults below deliberately, one at a time - in the workbench or in Packet Tracer. Before repairing, observe exactly what symptom it produces. In the following laboratories you will work the other way round: you will be given the symptom and will have to find the cause.

In the workbench, the second fault is produced like this:

R1# configure terminal
R1(config)# interface g0/0/1
R1(config-if)# shutdown
R1(config-if)# end

…and the repair is the same command with no in front. Notice that the objective "PC0 reaches Server0" goes out and comes back on by itself.

10Assignments

• Build the topology of figure 1 and configure it completely, according to the addressing plan
• Obtain a successful ping from every host to every other host (12 tests)
• Fill in the table of MAC and IP addresses on the four segments
• Reproduce the three faults and note the symptom of each, in your own words
• Add a third network, 192.168.3.0/24, on interface G0/0/2 of R1, with a switch and two hosts; verify complete connectivity
• Save the configurations and check with show startup-config

11Going further

12Self-check questions

13Deliverables

The .pkt file must also contain the third network required in the assignments, and the configurations must be saved in startup-config.