CCNA Network Infrastructure and Connectivity Practice Question
A network administrator is troubleshooting a connectivity issue between two routers, R1 and R2, connected via a serial link. R1 is configured with PPP encapsulation and CHAP authentication. R2 is configured with PPP encapsulation but no authentication. The link is down. Which statement explains the most likely cause?
⚠ Common exam trap
The trap here is assuming that CHAP requires identical hostnames or that PPP is incompatible with serial interfaces, rather than recognizing the authentication mismatch as the cause.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The link is down because R1 requires authentication but R2 is not configured to authenticate.
In PPP, if one side is configured with authentication (CHAP) and the other side is not, the authentication process will fail, and the link will not come up. R1 will send a challenge, but R2 cannot respond appropriately. Both ends must have compatible authentication configurations. The most likely cause is the mismatch in authentication settings.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The link is down because PPP encapsulation is not compatible with serial interfaces.
Why it's wrong here
PPP is a common encapsulation type for serial interfaces and is fully compatible. The issue is not with PPP encapsulation itself, but with the authentication mismatch. Serial interfaces commonly use PPP or HDLC. PPP supports authentication and is widely used. This statement is incorrect because PPP is compatible with serial interfaces; the problem lies in the authentication configuration.
- ✗
CHAP authentication failed because R2 did not send a challenge.
Why it's wrong here
CHAP is a three-way handshake where the authenticator sends a challenge. If R1 is configured with CHAP authentication, it will act as the authenticator and send a challenge to R2. R2, not having CHAP configured, will not respond correctly, leading to authentication failure. However, the statement says R2 did not send a challenge, which is incorrect because R1 sends the challenge. The failure is due to R2 not responding to the challenge, not because R2 didn't send one.
- ✗
The link is down because CHAP requires the same hostname on both routers.
Why it's wrong here
CHAP does not require the same hostname on both routers. Each router uses its hostname to identify itself during authentication, but they can be different. The authentication is based on a shared secret (password) and the challenge-response mechanism. The hostname is used to look up the password in the local database. Different hostnames are normal and expected. This statement is incorrect and does not explain the link failure.
- ✓
The link is down because R1 requires authentication but R2 is not configured to authenticate.
Why this is correct
When one side of a PPP link is configured with authentication (CHAP) and the other side is not, the authentication will fail. R1 will send a CHAP challenge to R2, but R2 will not have the necessary credentials or configuration to respond. As a result, the link will not come up. This is the most likely cause of the connectivity issue. Both ends must agree on authentication parameters for the link to establish.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.