350-401 Question #20: Real Exam Question with Answer & Explanation

Question

What is the difference between a RIB and a FIB?

Options

• AThe RIB is used to make IP source prefix-based switching decisions
• BThe FIB is where all IP routing information is stored
• CThe RIB maintains a mirror image of the FIB
• DThe FIB is populated based on RIB content

Explanation

RIB vs FIB Explained

Option D is correct because the RIB (Routing Information Base) acts as the master routing table, collecting routes from all routing protocols (OSPF, BGP, static routes, etc.), and the best routes from the RIB are then installed into the FIB (Forwarding Information Base), which is the optimized table actually used for packet forwarding decisions.

Why the distractors are wrong:

• Option A is incorrect because the FIB (not the RIB) is used to make forwarding/switching decisions on destination prefix, not source prefix
• Option B is incorrect because it's the RIB, not the FIB, that stores all IP routing information from multiple sources
• Option C is incorrect because the FIB is a derived subset of the RIB (containing only best paths), not a mirror image - they serve different purposes

Memory Tip: Think of it as a job application process - the RIB is the full stack of all applicants (routing sources), and the FIB only contains the hired candidates (best routes ready for work). The RIB feeds the FIB, never the other way around. RIB = Reservoir of routes; FIB = Forwarding in action.

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