TELE 3118: Network Technologies: Lab 2 [5 points]
This lab introduces you to Ethernet switching. You will form pairs
for this lab - each pair will have one PC and one switch, and two
pairs that are adjacent to each other will team up to perform most of
the experiments in this lab. You will be using the following tools in
The experiments you conduct in this lab are divided into three parts
described below. All functional components of this lab need to be
demonstrated to one of the lab demonstrators and initialled by
them. Answers to the questions in this lab should be written on this sheet of paper and handed in to the lab
- Ethernet switch: The "managed switch" we will use in this
lab is manufactured by Cisco Systems, one of the largest vendors of
Internet switching/routing equipment. The switch you will use is the
Catalyst 2940, which has 8 Fast-Ethernet ports (that operate at 100
Mbps full-duplex rate) and a Gigabit-Ethernet port (that you will not
use in this lab). The switch can be managed by connecting to it via a
serial cable and using the CLI (command-line interface) on a
hyper-term to issue commands and view configuration.
- Ethernet cables: You will bring the Ethernet cables you
built in your last lab. Each pair of students requires one
straight-through and one cross-over cable in this lab for interconnect
the various Ethernet devices.
- Wireshark: This is a packet capture tool already installed
on the lab PCs (an earlier version of Wireshark was called Ethereal),
and lets you capture and inspect packets that are being
transmitted/received on the Ethernet port of the PC. Please read the
Introduction to Wireshark document
prior to your lab session so you are familiar with its capabilities.
- Ping: This is a utility that lets you send packets to
another host and receive replies. To learn how to use ping invoke a
DOS window and type "ping /?". We will use this utility to check
connectivity between different PCs.
[1.5 point] Part I: Set-up and Connectivity: The objective of
this part is to set up your environment and establish basic
connectivity. The following steps are required:
If you demonstrate basic connectivity to your lab demonstrator, you
have obtained 1.5 points so far, and you may proceed to part
II. Otherwise, seek help from you lab demonstrator, who may reset your
switch using the CLI command "erase startup-config" and then power
cycle the switch.
- Log on to the PC (using account name "ciscolab"; password will be
supplied by lab demonstrator).
- Connect your Cisco switch to the power supply (do not power-on
- Using the serial cable (light blue), connect the serial port of
your PC's motherboard (your PC may have multiple serial ports, so
please make sure you use the serial port on the motherboard, which
will usually be blue in colour) to the console port at the back
of the switch.
- On your PC open a hyperterm (Start -> All Programs -> Accessories
-> Communications -> HyperTerminal), choose any name, connect using
"COM1" (which denotes the serial port), and set the bits per second to
9600 (leave everything else the same) - this should open a serial
terminal for you.
- Now power-on the Cisco switch. After a short while boot messages
should start appearing on your hyperterm. If not, seek help from the
lab demonstrator. Wait till the switch has booted, at which point it
should ask you if you want to enter the intial configuration dialog:
answer no and press enter.
- Now using a straight-through Ethernet cable connect the second
Ethernet port of your PC (NOT the one on the motherboard, but the one
that is closest to the side of the PC facing you) to port 1 of the
switch. The port light on the switch should go green after a while. If
not, seek help from the lab demonstrator.
- Wait till the pair adjacent to you have also obtained the green
light on their switch.
- Now interconnect your switch to theirs using the two
cross-through cables. Port 8 of your switch should be connected to
port 8 of their switch, and port 7 of your switch should connect to
port 7 of theirs. Wait till the port lights for both ports on both
switches go green. If that does not happen, seek help from your lab
- You now need to configure an IP address for the Ethernet port on
your PC. Click Start -> Control Panel -> Network Connections. Right
click on "Local Area Connection 2" and select properties. When a
window opens, scroll down to select "Internet Protocol (TCP/IP)",
and click on properties. Set the IP address to 192.168.1.x (where x is
your PC number marked as DELL-x on the front of your computer), mask
to 255.255.255.0, and leave the default gateway field blank. The IP
address settings take effect only after you have pressed OK to close
- Open a DOS command prompt and type "ipconfig" to verify your
IP address has taken effect.
- In your DOS window type "ping -t 192.168.1.x" (where x denotes
your neighbor's PC number) to check if there is connectivity between
your PC and your adjacent pair's PC. If not, seek help from the lab
demonstrator. To stop the ping traffic at any time you can press
[1.5 points] Part II: MAC address inspection: This part focuses
on Ethernet packet format and MAC addresses. You need to perform the
If you have answered the above questions correctly you should have
earned another 1.5 points, and you may proceed to part
III. Otherwise, seek help from you lab demonstrator.
- Determine the MAC address of the Ethernet port you are using on
your PC using "ipconfig /all", and note it down. Also note down the
corresponding MAC address of your adjacent pair's PC. [0.5 points]
- Open Wireshark, choose "capture -> interfaces" from the menu, and
select the (DLink) Ethernet interface connected to the switch.
This will start the packet capture (please ensure that the ping
session you started earlier is still going on, otherwise you will not
have packets to capture).
- Inspect any one of the ping ICMP packets captured by
Wireshark. Expand the "Ethernet II" header and note down the
source and destination MAC addresses: which PCs do they correspond to?
Also note (in hexadecimal) the Type field in the Ethernet header: what
payload is being carried by the Ethernet frame? [0.5 point]
- We will now use the hyperterm to inspect the mac address tables on
the switch. Using the hyper-term, enter administrative mode on the
Cisco switch by typing "enable". Using the "show mac-address-table"
command you can inspect the learnt MAC address table. Which port has
your switch learnt your PC's MAC address on? Which port has your
switch learnt your neighbor's PC MAC address on? [0.5 points]
[2 points] Part III: Spanning Tree Operation: This part helps
you see the operation of the Spanning Tree protocol. You need to
perform the following steps:
The objective of this lab is not just to get you to answer the
specific questions above but also to develop some comfort in working
with managed switches. If you have time available during your lab
session, please poke around the various commands available on the
hyperterm (use the "?" to obtain help and see all command completions
at any stage) to get an idea of what more could be done.
- Since the two switches are connected by two links, there is a
loop. Spanning tree should have broken the loop. Using the hyperterm
command "show spanning-tree vlan 1" first determine the bridge-id of
each switch. Now determine which of the two switches is the root. By
noting the STP states of the ports, determine the port that is blocked
by STP to break the loop. [0.5 points]
- Using Wireshark capture a BPDU on your PC, and examine its
contents. What is the bridge identifier of the switch that sent you
the BPDU? What is the root bridge according to the BPDU? Does it
correspond to the root you observed in the previous part? What is the
root path cost? What is the forward delay in the BPDU and what does it
mean? [0.5 points]
- We will now check how the spanning tree adjusts when a link
breaks. Of the two links connecting the two switches, disconnect one
of them (make sure that the ping session is still going on). If the
ping continues uninterrupted, reconnect the link, wait for a minute,
and disconnect the other link. Some ping packets should now be
lost while the tree recomputes. Time the duration for which
connectivity between the two PCs is lost (it should be restored
automatically after a while once the tree has been recomputed by STP).
Does the duration of loss of connectivity have any relation with the
"forward delay" value you noted in the previous part? Explain.
- Reconnect the broken link and wait for a minute. Now suppose the
switch that is the root is unreliable and we want to force the other
bridge to become the root. We can do this by configuring the "bridge
priority" of the other bridge to be lower than that of the current
root. To configure the switch you need to enter configuration mode by
typing "config terminal" and pressing enter. Typing "spanning-tree
vlan 1 ?" will show you how to go about configuring the bridge
priority. Once you have configured this type "exit" to get out of
configuration mode. Demonstrate that you have successfully changed the
root by showing the lab demonstrator the output of the "show
spanning-tree vlan 1" before and after your configuration. [0.5 points]