Introduction To ISDN, Part III: PAP
 by: Chris Bryant, CCIE #12933

Introduction To ISDN, Part III: Configuring PPP PAP Authentication

Now we know how the ISDN link comes up (interesting traffic), and some scenarios that might cause the link to stay up, we need to look at ISDN authentication schemes. The two methods Cisco certification candidates must be familiar with are PAP and CHAP.

Password Authentication Protocol (PAP) sends the username and password over the ISDN link in clear-text. Sending any passwords over any WAN link in clear-text is generally a bad idea, but it’s important to know you have this option.

Regarding both PAP and CHAP, it’s a common misunderstanding that each side must authenticate the other. PAP and CHAP both support bidirectional and unidirectional authentication; that is, R1 can authenticate R2 without R2 necessarily authenticating R1. It’s more common to use unidirectional authentication in a lab environment than a production network, but keep in mind that bidirectional authentication is an option, not a requirement.

The configurations of PAP and CHAP do have their similarities. For both, you’ll configure a username/password combination in global configuration mode. Newcomers to ISDN sometimes put the local router name in for the username; remember that the remote router name is the username.

The only real advantage of PAP over CHAP comes in the password configuration. Since PAP actually sends the password as a whole over the link, the two routers can send different passwords during authentication. The operation of CHAP requires that both routers use the same password, and we’ll see why in tomorrow’s article.

Under the BRI interface, you’ll enter encapsulation ppp and ppp authentication pap. So far, your authentication scheme looks like this:

username R2 password CCNA

Int bri0

encapsulation ppp
ppp authentication pap

PAP requires an extra command at this point. The ppp pap sent-username command is required under the interface, indicating the username and password this router will be sending to the remote router.

Int bri0

encapsulation ppp
ppp authentication pap
ppp pap sent-username R1 password CISCO

I always encourage CCNA and CCNP candidates to use as many debugs as possible when working in their lab, since these commands show us how things work. For any PPP authentication, always run debug ppp negotiation before sending interesting traffic to trigger the call. Watching exactly how PAP and CHAP work give you a much better understanding of what’s going on “behind the command”, and makes you a stronger candidate and a stronger networking engineer.

Tomorrow, we’ll take a look at CHAP, and why routers cannot use the same password on both ends of the link.

Keep studying!

Chris Bryant
CCIE ™ #12933
[email protected]

About The Author

Chris Bryant, CCIE (TM) #12933, has been active in the Cisco certification community for years. He worked his way up from the CCNA to the CCIE, and knows what CCNA and CCNP candidates need to know to be effective on the job and in the exam room.

He is the owner of http://www.thebryantadvantage.com, where he teaches CCNA and CCNP courses to small groups of exam candidates, ensuring they each receive the individual attention they deserve. Classes are offered over the Internet and in person in select cities. Chris has custom-written the Study Guide and Lab Workbook used in each course - no third-party training materials or simulators are used. You're invited to visit our site and check out our CCNA and CCNP courses and study aids, and to sign up for our weekly newsletter written personally by Chris. Chris is always glad to hear from Cisco certification candidates at [email protected]