CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
R1# show interfaces Serial0/0/0
Serial0/0/0 is up, line protocol is down
Hardware is GT96K Serial
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
DCE, no clock rate set
Last input never, output never, output hang never
Last clearing of "show interface" counters neverRefer to the exhibit. A network engineer is troubleshooting a connectivity issue between two routers connected via a serial link. The engineer runs the show interfaces Serial0/0/0 command on R1. Based on the output, what is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the distinction between 'interface is up, line protocol is down' (Layer 1 up, Layer 2 down) and 'interface is down, line protocol is down' (Layer 1 fault), tricking candidates into thinking a physical cable issue is the cause when the real problem is a missing clock rate on the DCE.
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
✓
The DCE end of the serial link is missing a clock rate configuration.
The output shows that the interface is up (line protocol is down), and the serial cable is physically connected. The absence of a clock rate on the DCE end of a serial link causes the line protocol to remain down because the receiving router cannot synchronize bit timing. Option D is correct because the DCE device must provide a clock signal for the serial link to establish Layer 2 connectivity.
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 interface is configured with the wrong encapsulation type.
Why it's wrong here
Encapsulation mismatch can cause line protocol down, but the output shows the default HDLC encapsulation with no indication of mismatch. Nothing in the exhibit suggests a mismatch, and the presence of the 'no clock rate set' message points to a different root cause.
- ✗
The interface is administratively shut down.
Why it's wrong here
If the interface were shut down, the status line would read 'Serial0/0/0 is administratively down, line protocol is down'. The actual output shows the interface is 'up', meaning it has been no shut and the physical layer is active.
- ✗
The serial cable is disconnected or has a physical fault.
Why it's wrong here
A disconnected or faulty cable results in a 'Serial0/0/0 is down, line protocol is down' status. Here, the interface is 'up', proving the cable is connected and the physical signal is detected. The issue is at Layer 2.
- ✓
The DCE end of the serial link is missing a clock rate configuration.
Why this is correct
The output explicitly states 'DCE, no clock rate set'. On a serial WAN link, the data communications equipment (DCE) must supply the clock signal. Without the clock rate command, the line protocol cannot come up, regardless of all other settings being 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 DCE end of the serial link is missing a clock rate configuration.Correct answer▾
Why this is correct
The output explicitly states 'DCE, no clock rate set'. On a serial WAN link, the data communications equipment (DCE) must supply the clock signal. Without the clock rate command, the line protocol cannot come up, regardless of all other settings being correct.
✗The interface is configured with the wrong encapsulation type.Wrong answer — click to see why▾
Why this is wrong here
Many candidates assume that a Layer 2 protocol down on a serial link is always caused by an encapsulation mismatch, overlooking the explicit clocking issue displayed in the output.
✗The interface is administratively shut down.Wrong answer — click to see why▾
Why this is wrong here
Novice engineers might misinterpret 'line protocol is down' as an indication that the interface is disabled, without reading the full status line.
✗The serial cable is disconnected or has a physical fault.Wrong answer — click to see why▾
Why this is wrong here
The trap: candidates see 'line protocol is down' and immediately think of a physical problem, missing the clear distinction that the interface itself is 'up'.
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?”
Go deeper
Related to this question
Learn chapter
ARP — Address Resolution Protocol
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
Key term
Bit
A bit is the smallest unit of data in computing and digital communications, representing a single binary value of either 0 or 1.
About these practice questions
This 200-301 question is part of Courseiva's 1,389-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
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.