CCNA Network Infrastructure and Connectivity Practice Question
Two routers, R1 and R2, are connected via a serial link. The interface on R1 shows 'Serial0/0 is up, line protocol is down' and no pings succeed across the link. You check the configuration and notice R1 has 'encapsulation ppp' but R2's serial interface was mistakenly left at the default encapsulation hdlc.
⚠ Common exam trap
Cisco often tests the distinction between 'line protocol is down' (Layer 2 issue) and 'Serial0/0 is down' (Layer 1 issue), and candidates mistakenly attribute a line protocol down state to physical problems or IP addressing errors rather than encapsulation mismatch.
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 mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.
The line protocol on a serial interface requires both ends to agree on the Layer 2 encapsulation type. R1 is configured with PPP (encapsulation ppp), while R2 defaults to HDLC. Since these encapsulations are incompatible, the routers cannot establish a valid Layer 2 connection, causing the line protocol to remain down despite the physical layer being up.
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 IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.
Why it's wrong here
This scenario misdiagnoses a Layer 3 issue as the cause of a Layer 2 state. The line protocol status reflects the data-link layer's ability to exchange frames and keepalives; IP addressing and subnetting do not influence it. If the interfaces were in different subnets, pings would fail because each router would not consider the other as directly reachable, but the serial interface would still show 'up, line protocol up' because Layer 2 is healthy. Since the question states the line protocol is down, the root cause must be below Layer 3, so an IP subnet mismatch is an incorrect explanation.
- ✓
The mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.
Why this is correct
This is the correct diagnosis. The serial interfaces are configured with different encapsulations: one uses PPP and the other uses HDLC (or another incompatible type). Each encapsulation defines a distinct frame structure, and a router receiving a frame that does not match its configured format cannot interpret the contents or properly validate the keepalive messages. Consequently, the keepalive process fails, and the router marks the line protocol as 'down' even though the physical layer (carrier detect) remains active. This is a classic Layer 2 mismatch that prevents the routers from ever establishing a working data-link connection.
- ✗
The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.
Why it's wrong here
This option incorrectly assumes a physical-layer problem. An interface's status is shown as 'Serial0/0 is up, line protocol is down', which explicitly indicates that the physical layer is functioning: the router detects a carrier signal and the interface is administratively up. If the serial cable were faulty or disconnected, the interface would instead display 'Serial0/0 is down, line protocol is down' or 'is down' for both, because the physical layer would not be able to provide a signal. Thus, a faulty cable does not explain a scenario where only the line protocol is down while the physical layer is up.
- ✗
The routers are missing a routing protocol configuration, so they cannot route traffic across the link.
Why it's wrong here
The absence of a dynamic routing protocol is irrelevant to the line protocol state. On a directly connected serial link, each router automatically installs a connected route for the subnet as soon as the interface is up and has an IP address; this occurs without any routing protocol. Even if a routing protocol like OSPF or EIGRP were configured, it would be dependent on the underlying data-link layer being operational. Since the line protocol is down, the interface cannot pass traffic at all, so adding or omitting a routing protocol has no effect on the observed symptom. The real problem remains the Layer 2 encapsulation mismatch.
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 mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.Correct answer▾
Why this is correct
This is the correct diagnosis. The serial interfaces are configured with different encapsulations: one uses PPP and the other uses HDLC (or another incompatible type). Each encapsulation defines a distinct frame structure, and a router receiving a frame that does not match its configured format cannot interpret the contents or properly validate the keepalive messages. Consequently, the keepalive process fails, and the router marks the line protocol as 'down' even though the physical layer (carrier detect) remains active. This is a classic Layer 2 mismatch that prevents the routers from ever establishing a working data-link connection.
✗The IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.Wrong answer — click to see why▾
Why this is wrong here
The line protocol down state is a Layer 2 indicator, not a Layer 3 problem. It reflects the failure of the data link layer to establish a connection, independent of IP addressing.
✗The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.Wrong answer — click to see why▾
Why this is wrong here
The 'interface up' part of the status explicitly confirms that the physical layer (Layer 1) is operational. A faulty cable would cause both physical and line protocol to show down.
✗The routers are missing a routing protocol configuration, so they cannot route traffic across the link.Wrong answer — click to see why▾
Why this is wrong here
Line protocol down indicates a Layer 2 failure, which occurs before any routing decision. Even without a routing protocol, the line protocol would come up if Layer 2 were functional, because the interface status is independent of routing.
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
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.
Key term
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it across a network.
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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.