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 this lab: 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 demonstrators.

[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:

  1. Log on to the PC (using account name "ciscolab"; password will be supplied by lab demonstrator).
  2. Connect your Cisco switch to the power supply (do not power-on yet).
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. Wait till the pair adjacent to you have also obtained the green light on their switch.
  8. 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 demonstrator.
  9. 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 both windows.
  10. Open a DOS command prompt and type "ipconfig" to verify your IP address has taken effect.
  11. 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 Ctrl-C.
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.

[1.5 points] Part II: MAC address inspection: This part focuses on Ethernet packet format and MAC addresses. You need to perform the following steps:

  1. 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]
  2. 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).
  3. 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]
  4. 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]
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.

[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:

  1. 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]
  2. 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]
  3. 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. [0.5 points]
  4. 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]
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.