Seeing CHAP authentication live Which Cisco IOS command displays the CHAP authentication process as it occurs between two routers?

Introduction / Context:
When troubleshooting PPP links, it is often necessary to observe authentication events in real time. CHAP (Challenge-Handshake Authentication Protocol) is supported over PPP and provides a secure challenge-response. Cisco IOS offers debug commands to watch the negotiation unfold, which is invaluable for diagnosing mismatched usernames, secrets, or hostname issues.

Given Data / Assumptions:

• The link uses PPP encapsulation.
• Routers are configured for CHAP authentication.
• We want to see the authentication conversation as it happens.

Concept / Approach:

The IOS command debug ppp authentication prints live messages showing CHAP challenges, responses, and results (success or failure). It also shows PAP if configured. show commands display static summaries and interface status but do not provide live, event-by-event details the way debug does.

Step-by-Step Solution:

Verification / Alternative check:

Use undebug all or no debug all to stop debugging after collecting evidence. For additional depth, debug ppp negotiation shows LCP/NCP events.

Why Other Options Are Wrong:

• show chap authentication / show ppp negotiation detail: not valid IOS commands.
• show interface serial 0: provides status, not live authentication exchanges.
• debug chap authentication: not the standard IOS keyword; CHAP events are included under debug ppp authentication.

Common Pitfalls:

• Running debugs on busy devices can be CPU intensive; use during maintenance windows or with care.
• For CHAP, the router's hostname and remote username must align; mismatches cause failures revealed by this debug.

Final Answer:

debug ppp authentication