The answer is a mismatch in PPP authentication credentials between the two routers. This is the most likely reason for PPP authentication failure because the Point-to-Point Protocol requires both ends of the link to share identical usernames and passwords—or, in the case of PAP, the exact same password—for the authentication phase to succeed. Even if the physical layer is up, a single mistyped character in the secret or a case-sensitive error will cause the link to reject the connection, leaving the interface in an "up/down" state. On the CCNA 200-301 v2 exam, this scenario tests your ability to isolate WAN troubleshooting issues after Layer 1 is confirmed working; a common trap is assuming the problem is a mismatched encapsulation type rather than a credential error. Remember the memory tip: "PAP and CHAP both need a perfect lap—one wrong character and the link takes a nap."
CCNA Practice Question: PPP CHAP authentication requires matching…
This 200-301 practice question tests your understanding of ip routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: pPP CHAP authentication requires matching credentials. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
R1#
interface Serial0/0/0
encapsulation ppp
ppp chap hostname Branch1
ppp chap password cisco123
R2#
interface Serial0/0/0
encapsulation ppp
ppp chap hostname Branch1
ppp chap password cisco321
Based on the exhibit, what is the most likely reason the PPP link is failing to authenticate?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The PPP authentication credentials do not match between the two sides.
The most likely reason is a credentials mismatch between the two sides of the PPP authentication setup. In practical terms, PPP authentication requires the peers to agree on the relevant identity and secret information. If those values do not align, the link may come up physically but authentication fails and the logical connection does not establish properly.
This is a very exam-relevant WAN troubleshooting scenario because it focuses on a realistic failure after the transport is already present.
Key principle: PPP CHAP authentication requires matching credentials
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 PPP authentication credentials do not match between the two sides.
Why this is correct
This is correct because the CHAP passwords shown are different.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
PPP CHAP authentication requires matching credentials
✗
The interfaces must use HDLC instead of PPP before authentication can work.
Why it's wrong here
This is wrong because PPP is exactly the encapsulation associated with this authentication scenario.
When this WOULD be correct
In a different scenario where a question specifies that a network is configured to use HDLC for link-layer encapsulation and requires authentication, a candidate might be asked about the compatibility of HDLC with authentication methods, making this option correct.
✗
CHAP requires the routers to be in different IPv4 subnets.
Why it's wrong here
This is wrong because subnet mismatch is not the issue shown here.
When this WOULD be correct
In a different exam scenario, a question might specify that CHAP authentication is being tested in a network where the routers are indeed in different IPv4 subnets, and the question could ask about the implications of such a configuration on CHAP's functionality. In this case, the option could be correct if it stated that subnet differences affect routing or connectivity, not CHAP itself.
✗
PPP authentication works only over GRE tunnels.
Why it's wrong here
This is wrong because GRE is unrelated to basic PPP CHAP authentication.
When this WOULD be correct
In a different question scenario where the context specifies that the authentication method is limited to GRE tunnels, and the question asks about the feasibility of PPP authentication in that specific setup, this option could be correct.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The PPP authentication credentials do not match between the two sides.Correct answer▾
Why this is correct
This is correct because the CHAP passwords shown are different.
✗The interfaces must use HDLC instead of PPP before authentication can work.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because PPP (Point-to-Point Protocol) does not require HDLC (High-Level Data Link Control) for authentication; PPP can authenticate independently of the encapsulation method used.
★ When this WOULD be the correct answer
In a different scenario where a question specifies that a network is configured to use HDLC for link-layer encapsulation and requires authentication, a candidate might be asked about the compatibility of HDLC with authentication methods, making this option correct.
Why candidates choose this
Candidates may find this option tempting because they might confuse the encapsulation methods and assume that HDLC is a prerequisite for any authentication process, leading to the misconception that PPP cannot function without it.
✗CHAP requires the routers to be in different IPv4 subnets.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because CHAP (Challenge Handshake Authentication Protocol) does not require routers to be in different IPv4 subnets for authentication to function. CHAP can authenticate devices regardless of their subnet configuration as long as they can reach each other over the network.
★ When this WOULD be the correct answer
In a different exam scenario, a question might specify that CHAP authentication is being tested in a network where the routers are indeed in different IPv4 subnets, and the question could ask about the implications of such a configuration on CHAP's functionality. In this case, the option could be correct if it stated that subnet differences affect routing or connectivity, not CHAP itself.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of how network protocols interact with IP addressing, leading them to incorrectly associate subnetting with authentication requirements.
✗PPP authentication works only over GRE tunnels.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because PPP (Point-to-Point Protocol) can operate independently of GRE (Generic Routing Encapsulation) tunnels; authentication can occur directly over a PPP link without requiring GRE.
★ When this WOULD be the correct answer
In a different question scenario where the context specifies that the authentication method is limited to GRE tunnels, and the question asks about the feasibility of PPP authentication in that specific setup, this option could be correct.
Why candidates choose this
Candidates may find this option tempting because they might confuse the functionalities of PPP and GRE, thinking that authentication mechanisms are tied to specific encapsulation methods, leading to misconceptions about their interoperability.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Be careful not to confuse physical link issues with authentication problems. Just because the link is up doesn't mean authentication is successful.
Trap categories for this question
Command / output trap
This is wrong because subnet mismatch is not the issue shown here.
Scenario analysis trap
This is wrong because PPP is exactly the encapsulation associated with this authentication scenario.
Detailed technical explanation
How to think about this question
PPP (Point-to-Point Protocol) supports authentication protocols like PAP and CHAP to validate peers before establishing a link. CHAP (Challenge Handshake Authentication Protocol) uses a three-way handshake where one router challenges the other to prove its identity using a shared secret password. Both routers must have matching CHAP usernames and passwords configured. If the credentials differ, the authentication fails, preventing the logical link from coming up despite the physical interface being operational. This is a common issue in WAN links where misconfigured passwords or usernames cause authentication failures. Unlike HDLC, which does not support authentication, PPP is designed to handle authentication natively. Also, CHAP does not depend on IP subnetting or GRE tunnels; it operates at the data link layer. Understanding this helps isolate authentication problems from other link or routing issues.
KKey Concepts to Remember
PPP CHAP authentication requires matching credentials
PPP supports authentication unlike HDLC
Authentication failure occurs despite physical link being up
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
PPP CHAP authentication requires matching credentials
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
IP Routing — This question tests IP Routing — PPP CHAP authentication requires matching credentials.
What is the correct answer to this question?
The correct answer is: The PPP authentication credentials do not match between the two sides. — The most likely reason is a credentials mismatch between the two sides of the PPP authentication setup. In practical terms, PPP authentication requires the peers to agree on the relevant identity and secret information. If those values do not align, the link may come up physically but authentication fails and the logical connection does not establish properly.
This is a very exam-relevant WAN troubleshooting scenario because it focuses on a realistic failure after the transport is already present.
What should I do if I get this 200-301 question wrong?
Review pPP CHAP authentication requires matching credentials, then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
PPP CHAP authentication requires matching credentials
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