Cisco Certified Network Associate (CCNA) Practice Exam

Which LCP function in PPP detects and disables looped links?

1. Magic number

2. Link-Quality Monitoring

3. Keepalive

4. Loop Detection Protocol

The correct answer is: Magic number

The correct answer is the magic number function within the Link Control Protocol (LCP) in Point-to-Point Protocol (PPP). The magic number is used primarily to identify looped links. It helps in detecting situations where the same data is being received from multiple sources due to network loops. When a link is detected as a loop, the magic number will help in determining the unique identifier of the link at both ends. If a mismatch in the magic numbers occurs, it signals a potential loop, allowing the system to take corrective action such as disabling the problematic link. Other functions mentioned, such as Link Quality Monitoring, keepalive, and features specifically intended for loop detection are important mechanisms within PPP but do not directly address the issue of looped links in the way that the magic number does. Link Quality Monitoring focuses on assessing the quality of the link and is not specifically aimed at detecting loops. Keepalive functions serve to maintain connections by sending periodic signals rather than focusing on topology issues.