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You need to know about the life of a packet for the CCNA exam. Do you know how MAC addresses are used? When they are used? How they change?

Get ready for the CCNA exam


Okay, so let’s see if we can answer the questions.
In the scenario we’ve been told that PC 1 pings PC t2 and we need to answer these questions based on the echo request message sent from PC 1 to PC 2.

So we firstly asked:
What is the destination MAC address in the frame at point A in the Network?
When traffic is sent from one layer to a network across the router, the PC is going to ARP for the routers MAC addresses and encapsulates the frame with the router’s MAC address when sending traffic to a remote destination.

So the MAC address that will be used is the MAC address of the router’s gigabit is 0.0.0 interface which is configured as this.
So the answer to the question is E Router 1’s gigabit 0.0 MAC address.

So I would say for the question one the answer is E, but let’s verify that by using simulation mode in packet tracer.

I’m going to edit my filters to only show up in ICMP traffic. You can click this option show all and then uncheck the various traffic types that you want to view.
On PC 1, I’m going to send a ping to the IP address of PC 2 and in the output here, I’ll be able to view the ICMP traffic.

So the inbound PDU shows us that the destination MAC address is the router’s MAC address which is configured as follows.
So again, answer to question 1 is the router is gigabit 0.0.0 address is the MAC address used as the destination MAC address in the frame and that address is this. So that would be the answer for question 2 which is asking us for the destination MAC address.

Question 3 asks us what is the encapsulation used?
Notice the encapsulation is Ethernet 2. So eth 2 is the encapsulation used
At point A in the network, we can see that again in the packet tracer capture.

We’re then asked to do something similar at point B in the network. So I’ll click capture forward. We could actually look at that under the frame on the switch and look at the outbound PDU. Notice the destination MAC address is the same Ether type is Ethernet 2. This is the MAC address of the router.

So the answers here would be very similar. We’re not asked in this example for the encapsulation used, but the encapsulation used is Ether 2.

So we could look at that here or we could look at the inbound PDU on the router and you’ll notice it has the same information. So that’s the answers to questions 1 to 5.

We then need to answer the question:
What is the destination MAC address at point C in the network? In other words, over here.

So looking at the outbound PDU, what you’ll notice is there is no MAC address.
The encapsulation used on the serial interface is HDLC. HDLC doesn’t use MAC addresses. MAC addresses are used in Ethernet. So we could see that here or as the inbound PDU on Router 2. So inbound PDU show something very similar. Notice the encapsulation used is HDLC, there is no MAC address.

So the answer to question 6 is, there is no MAC address at Point C in the Network and the encapsulation used is HDLC. We’re then asked similar information at pointed D in the Network. So what is the destination Mac address?
What is the actual MAC address and what is the encapsulation used at Point D?

So, D over here. So we could look at the router’s outbound PDU. MAC address is this is.
That is the MAC address of the PC.

So ipconfig
shows us the IP address of the PC

shows us the destination MAC address of the PC.

So this is the actual MAC address used. Going back to our questions, we need to specify whose MAC address is in the destination frame at point D that is PC 2 MAC address. This is the actual MAC address and we’re back on Ethernet which means it’s going to be Ethernet 2 as the encapsulation. We can verify that by looking at the outbound PDU on the router, notice it’s Ethernet 2.

So this is going to be Ethernet 2. Once again we could also if we wanted to look at the ingress traffic on the switch. Ingress shows something very similar. Destination MAC address frame type is Ethernet 2. That’s actually going to the same on this link. So on E, we could either look at the packet outbound on the switch. Notice it’s the same or we could look at the ingress PDU on the PC. Exactly the same information is shown for these questions.
So if we look at point E, the answer to the question will be the same, it’s PC 2’s MAC address. MAC address is that and the frame type is Ethernet 2. …

  • David Bombal