Technology Update

In part III of this ISDN primer, we learned that PPP has two main methods of authentication that Cisco certification candidates need to know how to configure: PAP and CHAP.

PAP has very few advantages over CHAP. PAP passwords are carried over the line in clear-text, which in today’s world is a very bad idea. PAP configuration also requires additional configuration with the “ppp pap sent-username” command, so anyone who can see your running configuration can also see the PAP password.

The only advantage PAP has over CHAP is a slim one. With PAP, a different password can be used by the each of the routers involved in the authentication. CHAP requires that the password be the same. Why? We’ll see as we examine CHAP authentication.

The First Step to Configuring CHAP

CHAP requires you to configure a username / password combination for any remote device that will be involved in authentication. (We’re assuming that the routers have already been configured with their names via the global hostname command.) Both routers will use the password CISCO.

R1:

username R2 password CISCO

int bri0

encapsulation ppp

ppp authentication chap

R2:

username R1 password CISCO

int bri0

encapsulation ppp

ppp authentication chap

Why CHAP Authentication Requires The Same Password On Both Routers

Remember how PAP sends the password over the line in clear-text? CHAP does not actually send the password over the line at all. Instead, CHAP runs a hash algorithm using the password and a random number. It is the result of this hash that is passed over the link. The remote router receives the hash result, and runs the exact same algorithm. If the result is the same, the authentication attempt will be successful. If the result is different, the authentication will fail. For this reason, the passwords must be the same.

Debug The Connection If Authentication Fails

Since two passwords are involved, the chances of one of the passwords being mistyped doubles. If you configure CHAP and the link dials but drops almost immediately, there’s an authentication problem. Run debug ppp negotiation and attempt to dial the line again. The output of this particular debug will show you where the problem is.

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There is an easy test that I like to use when evaluating a GPS, or any type of electronic device. I call it the user manual test. When I pick up a new GPS for the first time, I see how easily I can operate the GPS with common sense alone and without using the manual. Criteria for a good GPS is firmly based on how easily I can operate the menu options and pick up the functions. This is an aspect that some people overlook yet it is perhaps the single most important part of a GPS.

The first reason that user friendly functions and menus are so important is that the vast majority of people who buy GPS units are seasonal or occasional users. They are not out there 5 days a week, honing their skills and keeping the idiosyncrasies of their GPS at the forefront of their mind! Consequently, most users want to be able to pick the damn thing up and be able to get their hands on accurate information that can make the difference between getting lost, or even a fatal ending to an outdoor adventure.

The second reason that GPS units need to be easy to use is that they are often used by groups such as hunters, fishermen and hikers. This makes it quite likely that some members of the group will not be experienced with your particular model or brand of GPS. In these kinds of circumstances it is doubtful that anyone will want to sit down and read a user manual, that is if you still know where it is! So do yourself a favor and get a GPS that makes sense to you without having to go to the book, if for nothing else that from a safety point of view.

The third reason it that if you are willing to spend a couple of bucks on your GPS and choose a top brand such as Magellan or Garmin you don’t need to worry too much about what functions are included. The level of competition between these two companies is so high that neither is willing to put sub-standard products into the marketplace. At http://www.thegpscentre.com you can see reviews of GPS units and see how close these brand really are in both quality and price. What will make the difference will be the users ability to take advantage of the functions.

Remember, there is no point having a GPS with several navigation screens, thousands of automatic way-points if you can’t work out how to use them with ease. Happy trails and safe navigating to you all.

The author is a regular contributor to http://www.incargpsworld.com and permission to reproduce this article is given only on the basis that all links remain active and intact.

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