Home
>
Software
>
Introduction To ISDN, Part III: PAP

Introduction To ISDN, Part III: PAP

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

chris@thebryantadvantage.com

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 chris@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 chris@thebryantadvantage.com

cleaning service near Park Ridge ..

In The News:

Hotel TV privacy risks go beyond forgotten passwords when you sign into Netflix or other streaming apps on shared devices you don't control.

Apple's iOS 27 Visual Intelligence now lets your iPhone search objects directly on screen, but Samsung Galaxy and Google Pixel have a head start.

x47.c Windows malware uses xAI's Grok to maintain persistence on infected PCs while stealing passwords, browser cookies and API keys from victims.

Frankie LaPenna fought three robots in a cage, including two modified EngineAI T800 humanoids that can kick with up to 850 pounds of force..

MacSync malware hides malicious commands inside iCloud calendar events to download additional payloads onto infected Macs without raising suspicion.

Joseph Holycross dismissed his racing heart for nearly two years until his Samsung Galaxy Watch detected atrial fibrillation and changed everything.

Home title fraud begins when criminals forge your signature on a deed. Here is how to check your own property records and set up free alerts.

Jeffrey Ladish, a former Anthropic security team member, warns AI agents are colluding and hacking with no reliable solution to keep them in check.

Instinct Concierge and Meta Muse can now place phone calls to businesses on your behalf, but the privacy and data access tradeoffs are significant.

Olivia Moore spent $100 to create a fake RushTok influencer using ChatGPT and MiniMax, and thousands of Bama Rush fans couldn't tell the difference.

RatHat Android malware uses AI to steal banking credentials, intercept two-factor authentication codes and reconstruct your PIN from touch inputs.

The Fox News AI Newsletter covers the latest artificial intelligence technology advancements, including the challenges and opportunities AI presents now and for the future.

Apple and Google password managers can flag compromised, weak and reused passwords on your phone before credential stuffing attacks can strike.

Researcher Peter Garrigan says Moonshot AI's Kimi model was manipulated into providing instructions for biological weapons and assassination plans.

iPhone, Google Pixel and Samsung Galaxy phones now offer call screening that asks unknown callers to identify themselves before your phone rings.

Ransomware can lock your irreplaceable photos and documents in seconds. A tested backup strategy and stronger cybersecurity habits are your best defense.

ShinyHunters claims it breached FBIJobs.gov and stole Social Security numbers, home addresses and sensitive assignment details of FBI personnel.

Find My iPhone, Google Find Hub and Samsung Find have critical tracking settings to enable now, before your phone ever goes missing or stolen.

The FTC proposed a new enforcement policy on personalized pricing, warning companies that hide how personal data shapes the prices shoppers see.

The $250 million Apple Siri AI settlement is now accepting claims, with eligible iPhone owners potentially receiving up to $95 per device.

ShareAScan turns T-shirts into scannable digital business cards with QR codes that link to profiles the owner can update anytime they choose.

BragJack research reveals how a single malicious browser extension could hijack Gemini, Perplexity Comet and Edge AI agents with zero clicks.

A phishing campaign impersonating OpenAI uses fake ChatGPT billing emails with a convincing login page to steal credentials and payment details.

Startups are helping modernize America’s defense industry with new technology and innovative ideas as the War Department diversifies its inventory.

New account fraud victims jumped 31% in 2025, reaching 5.4 million. Criminals open accounts you may never see on your regular bank statements.

Introduction To ISDN, Part III: PAP

Software