Cisco CCNA Packet Tracer Ultimate labs: Exam Quiz Questions: Answers Part 3: Broadcasts & Collisions

Cisco CCNA Packet Tracer Ultimate labs: Exam Quiz Questions: Answers Part 3: Broadcasts & Collisions

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Can you answer the questions?

What are broadcast and collision domains. Can you prove the answers?

A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

In terms of current popular technologies, any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.

This is as compared to a collision domain, which would be all nodes on the same set of inter-connected repeaters, divided by switches and learning bridges. Collision domains are generally smaller than, and contained within, broadcast domains.

While some layer two network devices are able to divide the collision domains, broadcast domains are only divided by layer 3 network devices such as routers or layer 3 switches. Separating VLANs divides broadcast domains as well.

Translation:

In question 5, we’re told to assume that Switch 1’s MAC address is empty.
We can verify that by looking at the output of the show MAC address table command on the switch. As we can see here the MAC address table is empty.

We’re asked:
When PC 5 pings PC 8, what type of packet is sent to the switch initially and can we prove it? So, ipconfig on PC 5 shows us the IP address of PC 5 is 10.1.1.5
PC 8 has IP address 10.1.1.8
So what kind of frame or what kind of packet is sent to the switch?
When using terms such as frames and packets, once again, are we referring to layer 2 or layer 3 or layer 4 of the OSI model?

So what I’ll do on PC 5 is ping 10.1.1.8
Before I do that, notice the ARP cache is empty on PC 5. If it had just rebooted the ARP cache would be empty. So I’m going to send 2 pings into the network. We can see that the first packet that was generated is an ARP packet. Looking at the actual packet or frame, we can see that at layer 2 the frame has a destination address of a broadcast. The type of packet at layer 3 is ARP. So in the layer 3 headers we can see that this is an ARP packet requesting the MAC address of host with IP address 10.1.1.8

So the Ethernet type is 0x0806
In other words it’s an ARP packet.
Capture forward and before I continue the answer to question 5 is this is an ARP packet?
It’s a broadcast packet. We can see that again by looking at the inbound PDU on the switch. Notice destination address is a broadcast.

Who receives the packet because it’s a broadcast, it’s going to be flooded to the other devices in the network and then PC 6 and PC 7 are going to drop it because the packet is not destined to them. So the answer for question 6 is PC 6, PC 7 and PC 8 will receive the packet.

Now here’s where things change who receives the return packet?
So here we’ve got our ARP reply on the inbound PDU to the switch we can see that the target MAC address is this. That’s the MAC address of PC 5. So the MAC address is actually written into the frame. This is a unicast packet sent from PC 8 to PC 5. It’s not a broadcast unlike the ARP request.
So notice what happens now. The packet is only sent to PC 5. It’s not flooded out of all ports.
So the only PC that receives it is PC 5. That is different to our previous example where PC 1, PC 2 and PC 3 received the return traffic.

And notice the difference in question 8: When ping traffic is sent from PC 5 to PC 8, who receives it?
So here’s our ICMP requests or echo request message. We can see that its ICMP destination MAC address is PC 8, source MAC address is PC 5, source IP address is PC 5, destination IP address is PC 8. So notice now that the packet is only sent to PC 8. So that’s very different to what we saw when we were using a hub.

A switch is different to a hub in that, it has a separate collision domain on every port.
So when packets are sent from PC 5 to PC 8, they are sent directly between the devices. They don’t get flooded to the other PCs in the network. That is very different to a hub.

So, to prove that, what I’ll do is populate the ARP cache of PC 6.
So I’ll get it to ping PC 8 and I’ll run this in real time. So if we look at the ARP cache of PC 6 ARP cache is populated. The same is true on PC 5. So both PC 5 and PC 6 know the MAC address of PC 8….

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