350-401 Question #71: Real Exam Question with Answer & Explanation

Question

How are the Cisco Express Forwarding table and the FIB related to each other?

Options

• AThe FIB is used to populate the Cisco Express Forwarding table.
• BThe Cisco Express Forwarding table allows route lookups to be forwarded to the route processor
• CThere can be only one FIB but multiple Cisco Express Forwarding tables on IOS devices.
• DCisco Express Forwarding uses a FIB to make IP destination prefix-based switching decisions.

Explanation

CEF and FIB Relationship

Option D is correct because Cisco Express Forwarding (CEF) relies on the Forwarding Information Base (FIB) as its core data structure to make rapid Layer 3 switching decisions based on IP destination prefixes - the FIB is essentially part of CEF, pre-built from the routing table to enable fast, CPU-efficient packet forwarding without consulting the route processor each time.

Why the distractors are wrong:

• A is incorrect because the relationship is reversed - CEF builds and maintains the FIB from the routing table (RIB), not the other way around.
• B is incorrect because CEF was specifically designed to avoid forwarding lookups to the route processor; that describes process switching, which CEF replaces.
• C is incorrect because in reality, IOS devices can have multiple FIBs (e.g., one per VRF), making this statement factually backwards.

💡 Memory Tip: Think of CEF as a fast food restaurant - instead of cooking each meal from scratch (process switching), CEF pre-prepares everything using the FIB "menu," so packets are forwarded instantly without bothering the "head chef" (route processor). CEF uses FIB - CEF is the system, FIB is the lookup table.

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