CCNA IP Routing Practice Question
Exhibit
R1# show interface serial0/0/0 | include Encapsulation Encapsulation PPP R2# show interface serial0/0/0 | include Encapsulation Encapsulation HDLC
Based on the exhibit, what is the most likely reason the PPP link is down?
⚠ Common exam trap
A frequent exam trap is to assume that routing protocols such as BGP or OSPF must be configured before a PPP link can establish, or that PPP requires additional protocols like CAPWAP. This is incorrect because PPP operates at Layer 2 and must first establish the data link before any routing protocol can function. Another common mistake is to believe that serial links cannot use PPP and only support OSPF or other routing protocols, which confuses encapsulation with routing. The key is to recognize that mismatched encapsulation protocols like PPP versus HDLC prevent the link from coming up, regardless of routing configuration.
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 serial interfaces use different encapsulations.
The PPP link is down because the two ends are configured for different encapsulations. In practical terms, one side is using PPP and the other is using HDLC, so the devices are not speaking the same data-link protocol on the serial link. Until those encapsulations match, the link cannot come up correctly at the data-link layer. This is a classic WAN troubleshooting pattern that fits well into simulation-style exam coverage.
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 serial interfaces use different encapsulations.
Why this is correct
PPP and HDLC are distinct Layer 2 encapsulation protocols for serial WAN links. Cisco default is HDLC; if one end is configured with encapsulation ppp and the other remains HDLC, the link cannot negotiate LCP or exchange frames, so the line protocol stays down. This mismatch is the most likely cause because the physical layer (Layer 1) is up but the data link cannot establish.
- ✗
PPP requires CAPWAP on both routers.
Why it's wrong here
CAPWAP (Control and Provisioning of Wireless Access Points) is a protocol used for wireless LAN controller management, not for serial PPP. PPP uses LCP (Link Control Protocol) and NCP (Network Control Protocol) for establishment and configuration. CAPWAP has no role in PPP link establishment or operation.
When this WOULD be correct
In a different question scenario where the context involves wireless access points and their management, stating that 'PPP requires CAPWAP on both routers' could be correct if the question specifically addresses the need for CAPWAP to manage PPP connections in a wireless environment.
- ✗
The routers must run BGP before PPP can establish.
Why it's wrong here
PPP is a Layer 2 protocol that operates independently of any routing protocol. BGP is a Layer 3 path-vector protocol for exchanging routing information between autonomous systems. PPP can establish over serial links without any BGP configuration; in fact, routing protocols like OSPF or static routes can run over a PPP link.
When this WOULD be correct
In a different scenario where the question specifies that a PPP connection is being used to transport BGP routing information between two routers, and the exam asks about prerequisites for establishing that connection, this option could be correct if it stated that BGP must be configured first.
- ✗
Serial links can use only OSPF, not PPP.
Why it's wrong here
This confusion conflates the routing protocol OSPF with the data-link encapsulation PPP. PPP is a Layer 2 encapsulation that can carry any Layer 3 protocol, including IP, and any routing protocol such as OSPF, EIGRP, or BGP. Serial links support multiple encapsulations (HDLC, PPP, Frame Relay), and OSPF is not an encapsulation at all.
When this WOULD be correct
In a different scenario where the question specifies that only OSPF is allowed for routing over serial links, this option would be correct. For example, if the exam question stated that the network design mandates OSPF exclusively for serial connections, then this answer would be valid.
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 serial interfaces use different encapsulations.Correct answer▾
Why this is correct
PPP and HDLC are distinct Layer 2 encapsulation protocols for serial WAN links. Cisco default is HDLC; if one end is configured with encapsulation ppp and the other remains HDLC, the link cannot negotiate LCP or exchange frames, so the line protocol stays down. This mismatch is the most likely cause because the physical layer (Layer 1) is up but the data link cannot establish.
✗PPP requires CAPWAP on both routers.Wrong answer — click to see why▾
Why this is wrong here
CAPWAP (Control and Provisioning of Wireless Access Points) is a protocol used for managing wireless access points, not for serial WAN encapsulation. PPP is a Layer 2 protocol that operates independently of CAPWAP, so requiring CAPWAP for PPP is technically incorrect.
★ When this WOULD be the correct answer
In a different question scenario where the context involves wireless access points and their management, stating that 'PPP requires CAPWAP on both routers' could be correct if the question specifically addresses the need for CAPWAP to manage PPP connections in a wireless environment.
Why candidates choose this
Students might confuse CAPWAP with other control protocols or think that PPP requires some form of control protocol to operate, but PPP's control is handled by LCP and NCP, not CAPWAP.
✗The routers must run BGP before PPP can establish.Wrong answer — click to see why▾
Why this is wrong here
PPP is a Layer 2 protocol that establishes a direct link between two routers without requiring any Layer 3 routing protocol like BGP. BGP is an exterior gateway protocol used for routing between autonomous systems, and it has no dependency on PPP.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that a PPP connection is being used to transport BGP routing information between two routers, and the exam asks about prerequisites for establishing that connection, this option could be correct if it stated that BGP must be configured first.
Why candidates choose this
Some students might think that because BGP is often used over serial links, it must be required for PPP to work. However, PPP can operate with any Layer 3 protocol, including static routes, OSPF, or no routing at all.
✗Serial links can use only OSPF, not PPP.Wrong answer — click to see why▾
Why this is wrong here
Serial links can use multiple encapsulation types, including PPP, HDLC, Frame Relay, and others. OSPF is a routing protocol that runs over any Layer 2 encapsulation, including PPP. The statement that serial links can only use OSPF is factually incorrect.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that only OSPF is allowed for routing over serial links, this option would be correct. For example, if the exam question stated that the network design mandates OSPF exclusively for serial connections, then this answer would be valid.
Why candidates choose this
Students might think that because OSPF is commonly used on serial links, it is the only option. However, OSPF is a routing protocol, not an encapsulation, and PPP is a valid and widely used encapsulation for serial links.
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
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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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.