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Network Infrastructure and Connectivity →hardTroubleshootingObjective-mapped

Quick Answer

The correct answer is to configure R1’s GigabitEthernet0/0 with the commands 'speed 1000' and 'duplex full' to match R2, then clear the interface counters. This resolves the forced speed duplex mismatch fix because the show interface output reveals a classic mismatch: R1 is running at half-duplex, 100 Mb/s while accumulating 150 CRC errors in just over a minute—a clear sign that R2 is operating at full-duplex, likely at 1000 Mb/s. On the CCNA 200-301 v2 exam, this scenario tests your ability to interpret interface statistics and understand that CRC errors on a half-duplex link almost always indicate a duplex mismatch, not a cabling fault. A common trap is to assume autonegotiation will fix everything, but when one side is hard-coded, the other must match manually. Remember the memory tip: “Half-duplex + CRC = mismatch, not cable.”

CCNA Network Infrastructure and Connectivity Practice Question

This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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.

Exhibit

R1# show interfaces GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is ISR4321-2x1GE, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Internet address is 192.168.1.1/30
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is RJ45
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:01:23
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 1000 bits/sec, 2 packets/sec
  5 minute output rate 1000 bits/sec, 2 packets/sec
     512 packets input, 51200 bytes, 0 no buffer
     Received 512 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     150 input errors, 150 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     512 packets output, 51200 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

You are connected to R1. The link between R1 and R2 is experiencing intermittent connectivity and poor performance. Review the provided show interface output to identify the root cause(s) of the issue, then apply the necessary configuration changes to resolve the problem and restore full connectivity.

Output from R1: ``` GigabitEthernet0/0 is up, line protocol is up (connected) Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc) Internet address is 192.168.1.1/30 MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec) Half-duplex, 100Mb/s, link type is auto, media type is RJ45 output flow-control is unsupported, input flow-control is unsupported ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:01, output 00:00:01, output hang never Last clearing of "show interface" counters 00:01:23 Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue: 0/40 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 150 packets input, 1500 bytes, 0 no buffer Received 0 broadcasts (0 multicasts) 0 runts, 0 giants, 0 throttles 150 input errors, 150 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 0 multicast, 0 pause input 0 input packets with dribble condition detected 200 packets output, 2000 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out ```

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

  • Clue: "never"

    Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.

Question 1hardTroubleshooting
Study the full multicast explanation →

Exhibit

R1# show interfaces GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is ISR4321-2x1GE, address is aabb.cc00.0100 (bia aabb.cc00.0100)
  Internet address is 192.168.1.1/30
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is RJ45
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:01:23
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 1000 bits/sec, 2 packets/sec
  5 minute output rate 1000 bits/sec, 2 packets/sec
     512 packets input, 51200 bytes, 0 no buffer
     Received 512 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     150 input errors, 150 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     512 packets output, 51200 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

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

✓

Configure the interface with 'speed 1000' and 'duplex full' to match R2's settings, then clear counters.

The show interface output reveals that R1's GigabitEthernet0/0 is operating at half-duplex, 100 Mb/s, yet it is accumulating a high number of CRC errors (150 in 1 minute 23 seconds). This indicates a speed/duplex mismatch with R2, which is likely set to full-duplex at 1000 Mb/s. To resolve, you must manually configure R1 to match R2's proper settings by issuing the 'speed 1000' and 'duplex full' commands, then clearing the counters to start fresh monitoring. The other options are incorrect because they do not address the mismatch: replacing the cable would not fix a configuration issue; disabling autonegotiation alone may not fix the mismatch if the hard-coded values are still wrong; and increasing the MTU does not affect CRC errors caused by duplex mismatch.

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.

  • ✓

    Configure the interface with 'speed 1000' and 'duplex full' to match R2's settings, then clear counters.

    Why this is correct

    This is correct because CRC errors indicate a duplex mismatch; setting both speed and duplex manually ensures both sides match, resolving the issue.

    Clue confirmation

    The clue word "never" in the question point toward this answer.

    Related concept

    Authentication checks who the user is.

  • ✗

    Replace the faulty cable between R1 and R2 to eliminate CRC errors caused by physical layer issues.

    Why it's wrong here

    This is incorrect because CRC errors are more commonly caused by duplex mismatch than by faulty cables; the interface shows full-duplex, but the errors suggest the other end is half-duplex.

  • ✗

    Disable autonegotiation on the interface with 'no negotiation auto' to force the link to use the configured speed and duplex.

    Why it's wrong here

    This is incorrect because disabling autonegotiation alone does not resolve a duplex mismatch; you must explicitly set both speed and duplex to match the other device.

  • ✗

    Increase the interface MTU to reduce fragmentation and improve performance on the link.

    Why it's wrong here

    This is incorrect because MTU adjustments do not address CRC errors or duplex mismatches; CRC errors are layer 1/2 issues, not related to packet size.

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.

✓Configure the interface with 'speed 1000' and 'duplex full' to match R2's settings, then clear counters.Correct answer▾

Why this is correct

This is correct because CRC errors indicate a duplex mismatch; setting both speed and duplex manually ensures both sides match, resolving the issue.

✗Replace the faulty cable between R1 and R2 to eliminate CRC errors caused by physical layer issues.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that CRC errors are primarily a duplex mismatch symptom, not a cable fault, especially when the interface is up and errors appear quickly.

Why candidates choose this

Candidates pick this because CRC errors can be caused by bad cabling, but the context of intermittent performance and full-duplex setting points to mismatch.

✗Disable autonegotiation on the interface with 'no negotiation auto' to force the link to use the configured speed and duplex.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that disabling autonegotiation without setting speed/duplex can cause further mismatch or link failure.

Why candidates choose this

Candidates pick this because they know autonegotiation can cause issues, but they forget that manual configuration of both parameters is required.

✗Increase the interface MTU to reduce fragmentation and improve performance on the link.Wrong answer — click to see why▾

Why this is wrong here

The specific factual error is that MTU affects fragmentation and throughput, not error rates from duplex mismatch.

Why candidates choose this

Candidates pick this because they confuse performance issues with error causes, thinking larger MTU reduces overhead.

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: 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.

Trap categories for this question

  • Command / output trap

    This is incorrect because CRC errors are more commonly caused by duplex mismatch than by faulty cables; the interface shows full-duplex, but the errors suggest the other end is half-duplex.

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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

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 200-301 questions on access control and AAA configuration.

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FAQ

Questions learners often ask

What does this 200-301 question test?

Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Authentication checks who the user is..

What is the correct answer to this question?

The correct answer is: Configure the interface with 'speed 1000' and 'duplex full' to match R2's settings, then clear counters. — The show interface output reveals that R1's GigabitEthernet0/0 is operating at half-duplex, 100 Mb/s, yet it is accumulating a high number of CRC errors (150 in 1 minute 23 seconds). This indicates a speed/duplex mismatch with R2, which is likely set to full-duplex at 1000 Mb/s. To resolve, you must manually configure R1 to match R2's proper settings by issuing the 'speed 1000' and 'duplex full' commands, then clearing the counters to start fresh monitoring. The other options are incorrect because they do not address the mismatch: replacing the cable would not fix a configuration issue; disabling autonegotiation alone may not fix the mismatch if the hard-coded values are still wrong; and increasing the MTU does not affect CRC errors caused by duplex mismatch.

What should I do if I get this 200-301 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 200-301 questions on access control and AAA configuration.

Are there clue words in this question I should notice?

Yes — watch for: "never". Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.

What is the key concept behind this question?

Authentication checks who the user is.

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Last reviewed: Jun 6, 2026

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