CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
SwitchA# show interfaces gigabitethernet0/1
GigabitEthernet0/1 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 0011.2233.4401 (bia 0011.2233.4401)
Description: Link to SwitchB
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 10/100/1000BaseTX
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 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
1123 packets input, 123456 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
1123 packets output, 123456 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
SwitchA# show interfaces gigabitethernet0/1 counters errors
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize OutDiscards
Gi0/1 0 0 0 0 0 0
Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Gi0/1 0 0 0 0 0 0 0A network administrator is troubleshooting connectivity issues between two switches. Hosts connected to Switch A cannot ping hosts on Switch B. The link between the switches shows as up/up on both ends, but interface error counters (CRC, runts) are increasing rapidly. What should the administrator do to resolve the issue?
⚠ Common exam trap
Cisco often tests the misconception that a link showing 'up/up' means the physical layer is perfect, when in reality duplex mismatch causes high error rates while keeping the interface up, leading candidates to incorrectly suspect cable or hardware faults.
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
✓
Change the duplex setting on SwitchA's interface to auto-negotiation.
The increasing CRC and runts errors on an up/up link indicate a duplex mismatch, where one side is using full duplex and the other half duplex. Assuming SwitchB is using auto-negotiation (the default on Cisco switches), changing SwitchA's interface to auto-negotiation will allow both ends to negotiate the correct duplex setting, resolving the mismatch and stopping the errors.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Replace the faulty cable between the two switches.
Why it's wrong here
A faulty cable would typically produce physical-layer errors such as CRC errors, runts, giants, or late collisions, and these counters would be incrementing on the interface. Since the output shows the interface is up/up with no such physical-layer error counters, the cable integrity is not the issue. Duplex mismatches do not cause CRC errors; they cause late collisions and frame discards, which are resolved by adjusting duplex settings, not by replacing cabling.
- ✓
Change the duplex setting on SwitchA's interface to auto-negotiation.
Why this is correct
The correct fix is to change the duplex setting on SwitchA's interface to auto-negotiation. The interface is manually hard-coded to full-duplex, while the remote switch is likely auto-negotiating and has fallen back to half-duplex due to the negotiation failure. This duplex mismatch causes late collisions and poor performance. On 1000BASE-T, auto-negotiation is mandatory, so both ends must either auto-negotiate or be manually set to identical duplex and speed settings; changing SwitchA to auto allows both sides to agree on full-duplex.
- ✗
Replace the SFP module on SwitchA's GigabitEthernet0/1 port.
Why it's wrong here
A faulty SFP or optical module would manifest as physical-layer errors such as CRC errors, alignment errors, or link flaps, and the interface may even go down. The output shows the interface is operating normally at up/up status with no such physical-layer error statistics, indicating the SFP is not the cause. Duplex mismatch is a negotiation problem between the two switch interfaces, independent of the SFP hardware itself, so replacing the SFP would not resolve the issue.
- ✗
Enable the interface with 'no shutdown' command.
Why it's wrong here
The 'no shutdown' command is used to administratively enable an interface that is currently administratively down. If the interface were shut down, the status would display 'administratively down' rather than 'up/up'. Since the output already shows the interface as up/up, it is administratively enabled, and issuing 'no shutdown' would have no effect on the duplex mismatch that is causing the problem.
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.
✓Change the duplex setting on SwitchA's interface to auto-negotiation.Correct answer▾
Why this is correct
The correct fix is to change the duplex setting on SwitchA's interface to auto-negotiation. The interface is manually hard-coded to full-duplex, while the remote switch is likely auto-negotiating and has fallen back to half-duplex due to the negotiation failure. This duplex mismatch causes late collisions and poor performance. On 1000BASE-T, auto-negotiation is mandatory, so both ends must either auto-negotiate or be manually set to identical duplex and speed settings; changing SwitchA to auto allows both sides to agree on full-duplex.
✗Replace the faulty cable between the two switches.Wrong answer — click to see why▾
Why this is wrong here
Cable issues would manifest as input errors or interface flapping, but the counters show zero errors.
✗Replace the SFP module on SwitchA's GigabitEthernet0/1 port.Wrong answer — click to see why▾
Why this is wrong here
No errors are reported, so the SFP is functioning correctly.
✗Enable the interface with 'no shutdown' command.Wrong answer — click to see why▾
Why this is wrong here
The 'show interface' output clearly shows the interface is up, so no shutdown is not needed.
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?”
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Related to this question
Learn chapter
ICMP and Ping
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
Key term
Full duplex
Full duplex is a communication mode where data can be sent and received simultaneously between two parties, like a two-way conversation without interruptions.
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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.