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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.
Okay so let’s see if we can practically answer these questions.
To do that, I’m going to use simulation mode in packet tracer and I’m going to change the filters to only show up an ICMP messages. I don’t want to see all the other traffic types in the network.
So let’s see what happens when PC 1 pings PC4.
PC 1 has an IP address of 10.1.1.1
PC 4 has an IP address of 10.1.1.4
So we are going to send ICMP traffic. In other words ping traffic from PC 1 to PC 4 and then we’re going to observe what happens.
Now in this example, I’ll only send 2 packets, in other words two pings. Notice what happens in the network. There’s ICMP traffic but there is also an ARP frame that’s generated.
PC 1 needs to learn the MAC address of PC 4. So it sends out an ARP which is a broadcast requesting the target MAC address of PC 4.
So in answer to our first question: What type of packet is sent initially?
The answer is a ARP packet which is a broadcast packet.
We’ve proven that, by running simulation mode in packet tracer.
Now this is a broadcast and in this network, the devices are connected to a hub. So the traffic should be flooded to all devices in the network except the device that sent to the packet.
So who receives the packet?
PC2 does, PC3 does and so does PC4.
PC2 and PC3 will drop the packets because the ARP packet is not for their IP address.
PC2 as an example is configured with IP address 10.1.1.2
So PC2 is going to drop other packet. The ARP request is for IP address 10.1.1.4
So both PC2 and PC3 drop the packet but PC4 will accept the packet.
And what it should do now he sent back on ARP reply. So if we look at this packet, notice the inbound packet to the hub. It’s an ARP reply packet with the MAC address of PC4.
This is PC4’s IP address, this its MAC address. We can verify that by using the ipconfig command and I should say
Notice the MAC address which is the source MAC address of the frame and that’s the source MAC address we see over here.
So the next question is who receives the return traffic?
So if I click capture forward, notice PC1, PC3 and PC2 receive the return traffic.
So PC1 PC2 and PC3, PC2 and PC3 will drop the ARP reply because it’s not destined to them.
When we look at that packet, we can see that the destination MAC address is PC1 and we can confirm that by looking at the config of PC1……….
Notice the MAC address is this. So PC2 and PC3 will drop the ARP reply.
We’re then asked when ping traffic is sent from PC1 to PC4, who receives the traffic?
Now it’s a hub, so the traffic will be flooded.
So here’s the ICMP message.
We can see that it’s an Ethernet 2 frame using IP4 at Layer 3 and we can see it’s an ICMP packet and when we click capture forward, notice PC2, PC3 and PC4 receive the traffic. But again PC2 and PC3 will drop the packet because it’s not destined to them.
When we look at the inbound PDU or Protocol Data Unit on PC4, we can see that the destination IP address is PC4 and the destination MAC address is PC4.
We can verify that once again by using the ipconfig/all command and we can see the MAC address of PC4.
So that’s going to continue now.
When PC4 replies to PC1 the traffic is flooded and this is the problem with a hub even though the conversation is between PC1 and PC4, everyone in the network receives the traffic.