RIPv2 Loop-Prevention Toolkit — Choose the Mechanisms Used Which of the following mechanisms are used by RIPv2 to help prevent routing loops? (Items: 1) CIDR, 2) Split horizon, 3) Authentication, 4) Classless masking, 5) Holddown timers)

Introduction / Context:
RIPv2 retains distance-vector fundamentals while adding enhancements like classless routing and authentication. Loop prevention is still critical because DV protocols converge by sharing periodic updates rather than computing from a full topology database.

Given Data / Assumptions:

• We are considering RIPv2 features and general DV loop controls.
• Candidate mechanisms include split horizon, route poisoning, holddown timers, triggered updates, authentication, and classless support.
• We must select those specifically aimed at loop avoidance/damping.

Concept / Approach:
Split horizon suppresses sending a learned route back out the same interface, limiting two-node loops. Holddown timers temporarily ignore worse or conflicting updates after a route is declared down, damping count-to-infinity scenarios. CIDR/classless masking enable classless addressing and summarization (routing correctness/scale) but are not loop-prevention per se. Authentication protects update integrity, not loop mechanics.

Step-by-Step Solution:
Identify loop-focused controls: split horizon and holddown timers.Exclude CIDR/classless (addressing), and authentication (security), as not directly loop-preventing.Select “2 and 5.”Note: RIPv2 may also use poison reverse and triggered updates, though not listed here.

Verification / Alternative check:
Protocol references and certification guides list split horizon, holddown, poison reverse, and triggered updates among RIPv2 loop-control techniques.

Why Other Options Are Wrong:

• A/D/E: Include features that do not directly prevent loops (CIDR/classless, authentication).
• B: Mixes authentication with loop controls; authentication is orthogonal.

Common Pitfalls:
Assuming authentication prevents loops; it prevents unauthorized or tampered updates but not algorithmic loops.

Final Answer:
2 and 5