Internet Core Protocols: The Definitive Guide

Debugging ARP Problems

Since ARP is such a simple protocol, there aren’t many problems that can crop up with it. Most of the problems that do arise are exhibited elsewhere. You probably wouldn’t know that a problem was directly related to ARP unless you knew what some of the common symptoms were.

In addition, it is important to keep in mind that ARP can fail on its own, yet not appear to be an ARP-related problem. If the destination host specified in the original ARP request were down or otherwise unreachable, then it obviously would not receive (nor respond to) the original request. In such a case, the original sender would be unable to get a hardware address and would therefore be unable to send any IP packets to the destination device.

This is a substantially different concept from the host being able to send data to the recipient, even if that data is being ignored. While it may seem irrelevant (we can’t send data in either case), it is an important distinction.

If a host wanted to send data to another system, then it would first have to get the hardware address. If it could not get the hardware address, then no IP data would get sent at all. Under normal circumstances, it would be able to get the hardware address although IP delivery might still fail. An application should be able to tell the difference between these two types of failures, as they may have different ramifications for the end user.

For example, if you try to use the ping program to test a host that is powered ...