W2·A3 — Hub vs switch

Time: 35 min

Needs: 4 VPCS, an Ethernet hub, an Ethernet switch

GNS3 project: w02-a3-hub-vs-switch

Build it - start with a hub

Drag four VPCS and and one Ethernet hub into the canvas. Connect each VPCS to the hub.

Give them addresses:

# PC1 / PC2 / PC3 / PC4
ip 192.168.10.11/24        # .12 / .13 / .14
save

Confirm PC1 can ping the other three.

Predict first

If you listen on PC3's cable:

1. Can you hear the conversation between PC1 and PC2? ______
2. Same question, but with a switch in place of the hub. ______
3. Which layer does a hub operate at? ______
4. Which layer does a switch operate at? _____
5. Which topology does a hub implement? ______
6. Which topology does a switch implement? ______

Do this — listen on as a bystander

1. Right-click the PC3 cable → Start capture.
2. On PC1: ping 192.168.10.12
3. Read Wireshark.

What just happened

• PC3 was able to hear everything.
  • A hub is an electrical repeater — a signal arriving on one port is regenerated out of every other port, and it has no idea what a MAC address is.
  • We call this a broadcast domain: everybody receives everybody else's traffic.
  • We also call this a collision domain: only one host can usefully talk at a time
• A hub may physically look like a star topology, but it functions as a bus topology. In the early days of networking, nodes on a network communicated by tapping into a single cable and taking turns to transmit. The hub improved on this design by shortening the length of the cable, but kept the same functionality.

Do this — swap the hub for a switch

1. Delete Hub1. Browse Switches → drag an Ethernet switch on to the canvas and re-cable the VPCS.
2. Right-click on PC3's cable -> Start capture
3. On PC1: ping 192.168.10.12
4. Read Wireshark

What just happened

• PC3 see a little of the traffic, but not everything
  • it can see ARP request because they are broadcast
  • it sees neither ARP responses nor ICMP traffic. These are unicast, meaning a packet is sent to one and only one receiver and the response is sent to one and only one sender.
• This is still one broadcast domain because the switch forwards broadcast packets to every node
• It is no longer a single collision domain because the switch forwards unicast packets to the intended destination and no one else. This is what makes a switch a star topology
• The switch learns the location of each node on the network by reading inbound source MAC addresses

Record

Mark with a what PC3 sees after the first ping from PC1 to PC2

In one sentence: what does the switch know after the first ping from PC1 to PC2 that it did not know before?

If it went wrong