HELP WITH ARP REQUEST

bearfan

Is this a forum where you can post CCNA questions you have trouble understanding? If not, let me know.

This is a Transcenders practice test question I simply can't get correct.

In this scenario, you are downloading a file from the TFTP Server to the PC. What addresses
are present in the frames on the local network when the data returns from the TFTP Server?
(choose four).

There is a picture of 2 network separated by Router A, the Internet and Router B. The TFTP Server is on one network and the PC on the other.

A. IP address of the PC
B. MAC address of the PC
C. IP address of the TFTP Server
D. MAC address of the TFTP Server
E. IP and MAC address of the local routers ethernet port
F. IP and MAC address of the remote routers serial port
G. MAC address of both switch ports

I guess I don't fully understand exactly what information is in the ARP request at what point.
Can anyone shed some light?

Note: In this example the correct answers are A,B,C, and E. But I want to understand why. Also, there are other similar practice questions and it seems the answer to what's in the packet or frame can vary depending on where its at.

Webmaster

bearfan wrote:

Is this a forum where you can post CCNA questions you have trouble understanding? If not, let me know.

Well, yes and no. In general the answer is 'yes', you can post 'your' questions about the CCNA in this forum. But, questions from Transcenders, and other commercial as well as free practice exams providers are also copyright protected and cannot be duplicated without permission of the author. That includes posting in forums. The next time, try to bring the question down to the simple technical question it represents An easy way to do this is to place yourself in the situation as if it were a real-world scenario (a good thing to do regardless). The point is that we cannot host exact duplicates of anyone's practice questions but our own, so try to reword it in to a simple technical question to which you want the answer (or to your own scenario).

Anyway, no biggie. And to answer your question, it's not directly ARP related. The PC is already downloading frames from the TFTP server, so the ARP process took place already.

In general, with IP communication across networks the destination address will be the MAC + IP address of the PC and the source IP address will be the IP address of the original sender (i.e. the TFTP server) and the source MAC address will be the router's interface closest to the PC (which is typically the default gateway). So IP addresses stay the same when a packet cross networks (/routers, unless NAT is used), but the MAC address in the frame that hold the packet will change.

Pash

Just to add to Johan's post:

On that local router interface to the PC, if it isnt already in ARP cache the router will ARP broadcast to obtain the correct L2 destination.

Effectively it's exactly like:

OI! Whos got 192.168.1.50??? FF:FF:FF:FF:FF:FF (example)

Then the PC replies with an ARP reply, a unicast to the router interface who asked for the L2 address of 192.168.1.50.

I suggest if you have a network lab of somekind that you download:

http://www.wireshark.org/

This will give you a real look at network traffic and how the different protocols work.

Cheers,

Edit: Btw i dont like E as an answer in this question. Simply because it say's serial port rather than ethernet port, serial port's do not have MAC addresses. Very odd.....

bearfan

I got Wireshark and its very interesting.

Also, answer E should be "Ethernet" and not "Serial." That was a typo on my part. I changed it.

I'm scheduled to take the Intro test on July 23rd. I've been studying every day after work and on weekends, so I hope to be ready by then.

Pash

bearfan wrote:

I got Wireshark and its very interesting.

Also, answer E should be "Ethernet" and not "Serial." That was a typo on my part. I changed it.

I'm scheduled to take the Intro test on July 23rd. I've been studying every day after work and on weekends, so I hope to be ready by then.

Good stuff, I wish you luck mate.

Cheers,