352-001 Question #720: Real Exam Question with Answer & Explanation

The correct answer is C: per prefix. Loop-free alternate (LFA) pre-computes backup next-hops on a per-prefix and per-link basis, allowing immediate failover without triggering a full SPF recalculation.

Question

When designing fast convergence on a network using a loop-free alternate, on which two bases can the next-hop routes be precomputed? (Choose two.)

Options

Aper neighbor
Bper failure type
Cper prefix
Dper link
Eper network type

Explanation

Loop-free alternate (LFA) pre-computes backup next-hops on a per-prefix and per-link basis, allowing immediate failover without triggering a full SPF recalculation.

Common mistakes.

A. Per-neighbor is not a defined basis for LFA precomputation in RFC 5286 - LFA calculates alternate paths based on topological distances to prefixes and links, not on a per-neighbor granularity.
B. Per failure type is not a standard LFA computation basis - LFA addresses link and node failure scenarios through per-prefix and per-link calculations rather than categorizing by failure type.
E. Per network type is not a basis for LFA precomputation - the LFA algorithm operates on SPF-derived cost metrics and topology, not on OSPF network type classifications.

Concept tested. Loop-free alternate LFA precomputation bases for IP FRR

Reference. https://www.rfc-editor.org/rfc/rfc5286

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