CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
R1# show ip interface brief Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 192.168.1.1 YES manual up up GigabitEthernet0/1 unassigned YES unset administratively down down Serial0/0 10.0.0.1 YES manual up up Serial0/1 172.16.1.1 YES manual up down Serial0/2 unassigned YES unset administratively down down Loopback0 1.1.1.1 YES manual up up
Refer to the exhibit. A network engineer is troubleshooting a connectivity issue on R1. The serial link to R2 on interface Serial0/1 is using HDLC encapsulation, and the physical cable has been verified as good. The engineer has confirmed that the encapsulation type matches on both routers and that the clock rate is correctly configured on the DCE end. Based on the output, what is the most likely cause of the line protocol down state on Serial0/1?
⚠ Common exam trap
Cisco often tests the distinction between physical layer (line is up) and data link layer (protocol is down), leading candidates to incorrectly blame encapsulation or clock rate when the real issue is keepalive failure.
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 keepalive packets are not being received, causing the protocol to stay down.
The output shows that the line protocol is down while the physical layer (Serial0/1 is up). With HDLC encapsulation and clock rate confirmed correct, the most likely cause is that keepalive packets are not being exchanged between R1 and R2. HDLC uses keepalives to maintain the data link layer; if they are not received (e.g., due to a misconfiguration like 'no keepalive' on one side or a faulty cable), the protocol state will remain down even though the physical layer is operational.
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 encapsulation type is mismatched between R1 and R2.
Why it's wrong here
The scenario explicitly verifies that the encapsulation type is HDLC on both R1 and R2. If an encapsulation mismatch existed (e.g., HDLC versus PPP), the data-link frames would be unreadable, causing the line protocol to fluctuate or stay down, but that is not the case here. Because the encapsulation is confirmed matching, this option cannot be the cause of the line protocol being down.
- ✗
The clock rate has not been configured on the DCE end of the serial link.
Why it's wrong here
The clock rate command is only required on the DCE (data circuit-terminating equipment) side of a serial link to provide the timing signal for the line. The scenario confirms that the clock rate has been correctly configured on the DCE end, so the physical clocking is present. Without a clock signal, the line protocol would not come up, but that is ruled out by the confirmation.
- ✗
The IP address configured on Serial0/1 conflicts with another interface.
Why it's wrong here
An IP address conflict on Serial0/1 would cause Layer 3 reachability problems, such as intermittent routing or duplicate address warnings, but it would not affect the Layer 2 line protocol state. The line protocol is independent of IP addressing, so even with a conflict, the line protocol could remain up. Since the interface shows the line protocol as down, an IP address conflict is not a plausible explanation.
- ✓
The keepalive packets are not being received, causing the protocol to stay down.
Why this is correct
In HDLC serial links, routers exchange keepalive messages every 10 seconds by default, and the line protocol is declared down when successive keepalives are not received from the far end. Even though the physical layer is up (represented by the 'up' in 'Serial0/1 is up'), the protocol remains down because the local router has lost contact with the remote router at the data link layer. With encapsulation and clocking verified as correct, the most consistent cause is that keepalive packets are not being received, preventing the line protocol from transitioning to 'up'.
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 keepalive packets are not being received, causing the protocol to stay down.Correct answer▾
Why this is correct
In HDLC serial links, routers exchange keepalive messages every 10 seconds by default, and the line protocol is declared down when successive keepalives are not received from the far end. Even though the physical layer is up (represented by the 'up' in 'Serial0/1 is up'), the protocol remains down because the local router has lost contact with the remote router at the data link layer. With encapsulation and clocking verified as correct, the most consistent cause is that keepalive packets are not being received, preventing the line protocol from transitioning to 'up'.
✗The encapsulation type is mismatched between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
Candidates might fixate on the up/down status and assume encapsulation mismatch without considering the explicitly stated troubleshooting steps.
✗The clock rate has not been configured on the DCE end of the serial link.Wrong answer — click to see why▾
Why this is wrong here
Students often associate up/down with missing clock rate, but the question precludes this by confirming correct configuration.
✗The IP address configured on Serial0/1 conflicts with another interface.Wrong answer — click to see why▾
Why this is wrong here
Some learners confuse Layer 3 problems (IP issues) with the line protocol status that reflects Layer 2 health.
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?”
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
ARP — Address Resolution Protocol
Key term
Media Access Control
Media Access Control (MAC) is a sublayer of the Data Link Layer in networking that controls how devices on the same network share access to the physical medium and uniquely identifies each device with a hardware address.
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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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.