Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
The serial interfaces use different encapsulations.
This is correct because PPP on one side and HDLC on the other will prevent normal link operation.
B
Distractor review
PPP requires CAPWAP on both routers.
This is wrong because CAPWAP is unrelated to serial PPP encapsulation.
C
Distractor review
The routers must run BGP before PPP can establish.
This is wrong because PPP does not depend on BGP.
D
Distractor review
Serial links can use only OSPF, not PPP.
This is wrong because PPP is a standard serial WAN encapsulation.
Common exam trap
Common exam trap: answer the scenario, not the keyword
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.
Technical deep dive
How to think about this question
Point-to-Point Protocol (PPP) is a data link layer protocol used to establish a direct connection between two networking nodes over serial links. PPP encapsulates network layer protocol information over point-to-point links and supports authentication, compression, and multilink capabilities. In Cisco environments, PPP is commonly used on serial interfaces to provide a standardized encapsulation method that can interoperate across different vendors and equipment. For a PPP link to come up successfully, both ends of the serial connection must be configured to use the same encapsulation protocol. If one side uses PPP and the other side uses a different encapsulation, such as HDLC (High-Level Data Link Control), the link will fail to establish because the data link layer frames are incompatible. Cisco routers default to HDLC on serial interfaces unless explicitly configured otherwise. Therefore, mismatched encapsulation settings prevent the link from transitioning to the up state. A common exam trap is to confuse routing protocols or higher-layer dependencies with data link layer encapsulation issues. For example, some might incorrectly believe that routing protocols like BGP or OSPF must be configured before the link comes up, or that PPP requires unrelated protocols like CAPWAP. In reality, the link must first be operational at Layer 2 with matching encapsulations before any Layer 3 or routing protocol functions can occur. Practically, mismatched encapsulations cause the interface to remain down or in a down/down state, which is a fundamental troubleshooting step in WAN connectivity scenarios.
KKey Concepts to Remember
- PPP encapsulation provides a standardized Layer 2 protocol for serial links that supports authentication and multilink capabilities.
- Cisco routers default to HDLC encapsulation on serial interfaces unless PPP is explicitly configured on both ends.
- A serial link fails to come up if the two connected interfaces use different encapsulation protocols, such as PPP on one side and HDLC on the other.
- PPP must be configured on both ends of a serial link to establish a successful Layer 2 connection before any routing protocols can operate.
- Routing protocols like BGP or OSPF do not influence the establishment of the PPP link at Layer 2 and cannot bring up a link with mismatched encapsulation.
- CAPWAP is unrelated to PPP encapsulation and does not affect serial link operation or PPP establishment.
- Troubleshooting WAN links requires verifying matching encapsulation settings before examining routing or higher-layer configurations.
- Mismatched encapsulation causes the interface to remain down or in a down/down state, indicating a Layer 2 connectivity problem.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
PPP encapsulation provides a standardized Layer 2 protocol for serial links that supports authentication and multilink capabilities.
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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