The answer is an unsupported EAP method, such as EAP-TLS instead of PEAP, because the “invalid EAP code received” error on a Cisco switch during 802.1X troubleshooting indicates that the switch received an EAP packet with a code it cannot parse or forward. This occurs when the client initiates an authentication method the switch’s 802.1X state machine does not recognize, often due to a mismatch between the client’s configured EAP type and what the switch or RADIUS server expects. On the Cisco SCOR / CCNP Security Core 350-701 exam, this question tests your ability to differentiate between RADIUS-level errors (like shared secret mismatches) and switch-level EAP parsing errors, a common trap where candidates confuse authentication failures with network connectivity issues. Remember that the switch acts as an authenticator—it only relays EAP frames; if it cannot interpret the EAP code, the method is unsupported. Memory tip: “Code clash means method mismatch”—if the switch complains about the EAP code, look at the client’s EAP method first.
350-701 Practice Question: Secure Network Access, Visibility and Enforcement
This 350-701 practice question tests your understanding of secure network access, visibility and enforcement. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
A network administrator is configuring 802.1X for wired access on a Cisco switch. The switch is configured for RADIUS using a Cisco ISE server. During testing, a client that supports 802.1X is unable to authenticate and fails to gain network access. The administrator checks the switch logs and sees "Authentication failed: invalid EAP code received". What is the most likely cause?
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 client is using an unsupported EAP method (e.g., EAP-TLS instead of PEAP).
Option C is correct because the error "invalid EAP code received" indicates that the switch received an EAP packet with a code it does not support, typically due to an unsupported EAP method. Option A is wrong because a shared secret mismatch would produce a different RADIUS error. Option B is wrong because trunk port configuration would cause VLAN issues, not EAP parsing errors. Option D is wrong because RADIUS unreachability would cause timeouts or no response.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 client is using an unsupported EAP method (e.g., EAP-TLS instead of PEAP).
Why this is correct
The switch cannot process an unrecognized EAP code, which occurs when the client negotiates an unsupported method.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
The RADIUS server is unreachable.
Why it's wrong here
Unreachable RADIUS server results in timeouts or no response, not an invalid EAP code from the switch.
✗
The switch is configured with the wrong shared secret for RADIUS.
Why it's wrong here
Shared secret mismatch results in RADIUS authentication failures with different error messages, not EAP code errors.
✗
The switch port is configured as a trunk port rather than an access port.
Why it's wrong here
Trunk port configuration would cause VLAN tagging mismatch, not an EAP code error at the authentication layer.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-701 questions on access control and AAA configuration.
Secure Network Access, Visibility and Enforcement — This question tests Secure Network Access, Visibility and Enforcement — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The client is using an unsupported EAP method (e.g., EAP-TLS instead of PEAP). — Option C is correct because the error "invalid EAP code received" indicates that the switch received an EAP packet with a code it does not support, typically due to an unsupported EAP method. Option A is wrong because a shared secret mismatch would produce a different RADIUS error. Option B is wrong because trunk port configuration would cause VLAN issues, not EAP parsing errors. Option D is wrong because RADIUS unreachability would cause timeouts or no response.
What should I do if I get this 350-701 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-701 questions on access control and AAA configuration.
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?
Authentication checks who the user is.
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