Answers to Latest Question: ARP, InARP, EIGRP

This answer to a #CCNA question summarizes some key points about ARP and inverse ARP, and makes you break down what is needed to support a particular packet. It really helps to draw the steps and the packet, so make sure to do so. Feel free to ask questions!

• • • • • • • Original question
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Answers

B

Explanation: Answers A and E

Two answers, A and E, relate specifically to IP ARP on a LAN. In the first case, it mentions R1’s potential ARP entry listing info about host PC1. R1 needs that entry, but only to forward IP packets to PC1’s IP address; the message listed in the question is going towards PC3, so that ARP table entry isn’t needed for that particular packet. As for potential ARP entry on PC3 listing PC1’s IP and MAC, ARP always lists MAC addresses on the local subnet, so PC3’s ARP table would never list PC1’s MAC. PC3 would instead need an ARP entry for its default gateway, R3.

Explanation: Answer C

One of the answers, C, lists information about EIGRP. In this topology, EIGRP must work between the four routers. Additionally, the routers must advertise about the subnets connected to the LANs. However, the routers do not have to send EIGRP Hellos onto the LANs before the packet listed in the question could flow from PC1 to PC3. In this case, R3’s EIGRP configuration could have made its LAN interface passive, and never sent an EIGRP Hello, and the packet listed in the question would still have worked.

Explanation: Answer B (Correct)

Finally, for the correct answer B, router R1 needs to know mapping information that maps R3’s Frame Relay IP address (10.0.4.3) to R1’s local DLCI (103). The problem statement mentioned that no static mapping was used, and there are only two ways to know that mapping on a multipoint subinterface: static mapping and inverse ARP. So, before the ICMP Echo packet could flow, R1 must have heard an Inverse ARP message from R3 such that R1 knew to map address 10.0.4.3 to R1’s local DLCI 103.

Explanation: Answer D

For the other answer (D) that lists Inverse ARP, R3 might benefit from knowing a mapping of R4’s Frame Relay IP address and the matching local DLCI, but that’s not necessary for the packet flowing from PC1 to PC3.