Applied ARP

By certskills February 14, 2017 12:05

This next question continues my recent series of sample questions, all of which related in some way to the new video content in the recently-released CCENT Exam Prep LiveLessons video product. Specifically, this question asks about ARP, and gets at the heart of the biggest issue with ARP: what ARP entries do devices need to request and learn in support of normal packet flow in a network? As usual, I’ll break the QA into a question post (this one) and a follow-up answers post.

The Question

In the figure, all routers perform IPv4 routing, and all switches are layer 2 switches. Host A successfully pings server B. Immediately afterwards, an engineer issues a show arp command on router R2. Which answers describe an ARP table entry that should exist in the output from this command for the purpose of supporting the packets created as a result of the ping command?

A) An entry for host A’s IP + MAC address

B) An entry for server B’s IP + MAC address

C) An entry for one of router R3’s IP + MAC addresses

D) An entry for one of SW2’s IP + MAC addresses

E) An entry for one of R1’s IP + MAC addresses

Figure 1: The Network for the ARP Question


More Background and Practice with CCENT EPL

As mentioned in the intro, I chose to write this particular question because of the release of CCENT EPL product. In fact, the figure from this question is identical to the one used in CCENT EPL video 13.3, another sample question, titled “End-to-End Data Plane Forwarding”. That question requires you to think about a variety of topics related to how network devices forward messages, including how ARP works.

Also, note that CCENT EPL includes one Common Mistake video (8.1, “Applying ARP in Enterprise Networks”) which speaks directly to the kinds of issues you must understand before you can answer today’s blog question with confidence.


Answer Post in a Few Days!

I’ll post an answer post in a few days. Watch for it! It should be the next post in chronological order as well, so watch for the link at the bottom of this page.

EIGRP Enabler #1 - Answers
Applied ARP - Answers
By certskills February 14, 2017 12:05
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February 14, 2017 8:40 pm

I think when the Client sends the packetm it must be Server’s B IP since L3 addressing wont change along the path, but the Mac would be of the SW2 who’s towards the Server. For the return traffic, it would be Host A’s IP with R1’s Mac (towards the Client).

Reply to  ninjasd
February 15, 2017 10:15 am

So, what’s in R2’s ARP table that’s necessary to support the ping messages? That’s the crux of the question.
Answer post is scheduled for tomorrow…

Reply to  lyjo
February 16, 2017 11:50 am

Option E: (IP + MAC addresses) of Router 1, which is the learned MAC from the directly connected link. Not SW2’s IP because that shows like a L2 hop.

February 15, 2017 12:36 pm

C and E, I am pretty sure.

In the R2’s ARP table normally we must find also the R2’s IP + MAC.addresses

February 15, 2017 1:16 pm

A and C

February 15, 2017 1:25 pm

c and d

February 15, 2017 1:26 pm

sorry c and e

February 16, 2017 1:31 am

C and E.