CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1. The network team reports intermittent connectivity between R1 and R2. Examine the following `show interface` output from R1 to identify the root cause of the issue. Then apply the necessary configuration commands on R1 to resolve the problem.
GigabitEthernet0/0 is up, line protocol is up Internet address is 10.0.0.1/30 MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec) Full-duplex, 100Mb/s, media type is RJ45 output flow-control is unsupported, input flow-control is unsupported Last input 00:00:01, output 00:00:01, output hang never Last clearing of "show interface" counters never 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 100 packets input, 8000 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 100 input errors, 100 CRC, 0 frame, 0 overrun, 0 ignored 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out
⚠ Common exam trap
Do not assume that manually setting speed and duplex is always correct. CRC errors often point to a duplex mismatch, and the standard fix is to enable auto-negotiation on both ends. Also, remember that 'no shutdown' is for administratively down interfaces, not for error conditions.
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
✓
Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.
The show interface output reveals 100 CRC errors and 100 input errors, indicating a physical layer problem. Additionally, the interface is manually configured for full-duplex and 100 Mb/s. CRC errors often point to a duplex mismatch or faulty cable. Since R1 is set to full-duplex, if R2 is set to auto-negotiation (or half-duplex), a mismatch occurs. The fix is to set both sides to auto-negotiation by removing manual speed and duplex settings. On R1, enter interface configuration mode and issue 'no speed' and 'no duplex' to revert to auto-negotiation, then verify with 'show interfaces' that the interface negotiates to full-duplex and errors stop incrementing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.
Why this is correct
This is correct because CRC errors and input errors often indicate a duplex mismatch. Manually setting speed and duplex on one side while the other uses auto-negotiation can cause such errors. Removing manual settings allows both sides to auto-negotiate, resolving the mismatch.
- ✗
Enter interface configuration mode and issue the 'speed 100' and 'duplex full' commands to ensure both sides are set to the same values.
Why it's wrong here
This is incorrect because the interface is already manually set to speed 100 and duplex full, yet errors persist. Reapplying the same settings does not address the root cause; the problem is likely a mismatch with the remote side.
- ✗
Enter interface configuration mode and issue the 'no shutdown' command to bring the interface up.
Why it's wrong here
This is incorrect because the interface is already administratively up and line protocol is up; the 'show interface' output indicates the physical layer is active, not in an administrative shutdown state. The 'no shutdown' command only changes the administrative state from down to up and does nothing to alter speed, duplex, or clear CRC/input errors. Since the errors persist despite the interface being operational, the root cause is a duplex mismatch, not a shutdown state, so this command would have no effect.
- ✗
Enter interface configuration mode and issue the 'duplex half' command to match the remote side's duplex setting.
Why it's wrong here
This is incorrect because setting duplex to half on R1 would create a mismatch if R2 is using full-duplex (which is typical for 100 Mb/s links). The correct approach is to auto-negotiate, not force half-duplex.
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.
✓Enter interface configuration mode for the problematic interface and issue the 'no speed' and 'no duplex' commands to revert to auto-negotiation.Correct answer▾
Why this is correct
This is correct because CRC errors and input errors often indicate a duplex mismatch. Manually setting speed and duplex on one side while the other uses auto-negotiation can cause such errors. Removing manual settings allows both sides to auto-negotiate, resolving the mismatch.
✗Enter interface configuration mode and issue the 'speed 100' and 'duplex full' commands to ensure both sides are set to the same values.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that reapplying the same manual settings does not resolve a duplex mismatch; it may even perpetuate the mismatch if the remote side is set to auto-negotiation.
Why candidates choose this
Candidates might think that ensuring both sides have the same manual settings will fix the issue, but they overlook that the remote side might be using auto-negotiation, which requires both sides to auto-negotiate.
✗Enter interface configuration mode and issue the 'no shutdown' command to bring the interface up.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that 'no shutdown' is used to enable an interface that is administratively down, but the interface is already up.
Why candidates choose this
Candidates might assume that intermittent connectivity is due to the interface being down, but the show interface output would show 'administratively down' if that were the case.
✗Enter interface configuration mode and issue the 'duplex half' command to match the remote side's duplex setting.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that forcing half-duplex is unlikely to resolve the issue; it may worsen performance and still cause errors if the remote side is full-duplex.
Why candidates choose this
Candidates might think that since CRC errors indicate a duplex mismatch, setting both sides to half-duplex would fix it, but they ignore that the remote side might be set to full-duplex or auto-negotiation.
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?”
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
Registered Jack Function 45
An RJ45 connector is the clear plastic plug at the end of an Ethernet cable that connects computers, routers, and switches to form a wired network.
Key term
Auto-negotiation
Auto-negotiation is an Ethernet feature that allows two connected devices to automatically agree on the best possible speed and duplex mode for their link.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.