Is this book correct?

Question. On an Ethernet network, PC1 sends a message to PC2. The message must cross two routers along the pathway. Which of the following statements are true concerning the communication between PC1 and PC2?

A. The frame header changes twice during the time it is sent from PC1 to finally reaching its destination at PC2.

B. The frame header changes three times during the time it is sent from PC1 to finally reaching its destination at PC2.

C. The packet header changes at each router in the delivery path.

D. The packet header never changes during the delivery path.

E. PC1 broadcasts a DNS message to determine PC2’s MAC address before building the frame.

F. PC1 broadcasts an ARP request to determine PC2’s MAC address before building the
frame.

Answer: I chose B and D, but the book also suggests F. I had the understanding that once a PC determines that a host is not on it's local network then it immediately ARPs for it's default gateway to send the packet for delivery.

The book's explanation is: Finally, PC1 broadcasts an ARP message for PC2’s IP address, looking for a MAC. The router will respond.

Can anyone clear this up for me?

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Comments

mikej412

What's the book?

Forsaken_GA

Well, I'll help you work through this -

What is your reason for taking B over A for the first part of the question? How did you arrive at three frame header changes instead of two?

You're correct on D, the source IP and the destination IP will remain the same, end to end (assuming there's no NAT involved)

And F can also be correct. Assuming PC1 doesn't have an ARP entry for PC2, it will ARP for it ahead of time, and the router will respond to proxy for the host on the other subnet.

nevolved

PC1 shouldn't ARP for PC2's MAC address. As soon as PC1 compares its network address with that of PC2 it will see that it is on a seperate network (run a route print on your Windows box). It would ARP for the MAC of Router1(DG) if it had not been seen.

Forsaken_GA

nevolved wrote: »

PC1 shouldn't ARP for PC2's MAC address. As soon as PC1 compares its network address with that of PC2 it will see that it is on a seperate network (run a route print on your Windows box). It would ARP for the MAC of Router1(DG) if it had not been seen.

For most cases, that's correct, but if you're in a situation where proxy arp is in use for some reason (ie, same subnet over two different physical segments), then PC1 would indeed have to arp for PC2 first. It's sort of a trick question, but it is possible that an ARP request/reply is necessary before you can send to a host across two routers (I should have used the word segment instead of subnet in my previous response)

Agent6376

The book is published by McGraw Hill, and it's named CCENT Study Guide, written by Matt and Angela Walker.

My reason for answering B is this:

1.When PC1 sends data to PC2 the frame will have the source Mac of PC1 and a destination Mac of the router.

2. The first router receives the data and strips the l2 header/trailer, checks the destination IP, realizes that it's not meant for the router, then checks its routing table on what's the best route to send the traffic to. It then encapsulates its on l2 header/trailer with a source Mac of it's own Mac address and a destination Mac address of the nearby router that knows the route to PC2.

3. The second router receives the data and again strips the l2 header/trailer, checks the destination IP and sees that the host is on its local network. The router ARPs for the host's Mac address if it isnt in the Mac Address table already, then creates a l2 header/trailer with the source being the router and the destination being the host.

I see where you're going with this question, I think. The Mac address goes through three different stages, but only technically changes twice: once when it hits the first router, then another when it hits the second.

However, wouldn't it be fair to say that F is incorrect as proxy ARP is outside of the scope of CCENT. Though I understand what it is, it hasn't been taught in any of the readings or videos that I've seen.

Thanks for the responses!

Forsaken_GA

Agent6376 wrote: »

However, wouldn't it be fair to say that F is incorrect as proxy ARP is outside of the scope of CCENT. Though I understand what it is, it hasn't been taught in any of the readings or videos that I've seen.

Well, that's largely dependent on the opinion of the person who wrote the question hehe. I'd say yes, but it may be one of those things the author feels is important for you to be at least be aware of, if not learn outright. (I absolutely hate proxy arp, for the record)

Agent6376

Forsaken_GA wrote: »

Well, that's largely dependent on the opinion of the person who wrote the question hehe. I'd say yes, but it may be one of those things the author feels is important for you to be at least be aware of, if not learn outright. (I absolutely hate proxy arp, for the record)

I'd contend that he or she should have mentioned it in the explanation of the answer, but regardless I appreciate you taking the time to answer my questions. I'm sitting the CCENT tomorrow at 2pm CST so wish me luck!