352-001 Question #296: Real Exam Question with Answer & Explanation

The correct answer is A: Loop-Free Alternates. Loop-Free Alternates (LFA) provide pre-computed IP fast reroute backup paths that activate immediately upon link failure, enabling sub-60ms unicast convergence without triggering a new SPF calculation.

Question

Refer to the exhibit. You are asked to design this OSPF network to converge within 60 ms for unicast packets after a topology change due to a single link failure. Which technology can be enabled while conforming to the design requirements?

Exhibit

Options

ALoop-Free Alternates
BBFD
CIGP neighbor timer tuning
DIGP SPF timer tuning
ERSVP-TE Fast Reroute

Explanation

Loop-Free Alternates (LFA) provide pre-computed IP fast reroute backup paths that activate immediately upon link failure, enabling sub-60ms unicast convergence without triggering a new SPF calculation.

Common mistakes.

B. BFD rapidly detects link failures but does not itself provide alternate forwarding paths - the router still requires SPF recalculation after BFD detection, which can exceed 60ms.
C. Tuning IGP hello and dead timers reduces failure detection time but convergence still depends on SPF execution, making consistent sub-60ms convergence difficult to guarantee.
D. SPF timer tuning reduces SPF scheduling delay but does not eliminate the SPF recalculation step after a failure, and reliably achieving sub-60ms convergence through SPF tuning alone is not practical.
E. RSVP-TE Fast Reroute can achieve 50ms reroute but requires full MPLS Traffic Engineering infrastructure, which is not indicated as present in this plain OSPF network design.

Concept tested. OSPF IP fast reroute using Loop-Free Alternates

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

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