352-001 Question #45: Real Exam Question with Answer & Explanation

The correct answer is A: to prevent traffic loss when the path from a router to a BGP next hop traverses another router that. Setting the IS-IS overload bit or advertising an infinite OSPF metric prevents other routers from forwarding transit traffic through a router whose IGP has converged but whose BGP table is not yet fully populated, avoiding black holes.

Question

Why might you want to synchronize IGP and BGP convergence by advertising an infinite metric in OSPF or setting the overload bit in IS-IS?

Options

Ato prevent traffic loss when the path from a router to a BGP next hop traverses another router that
Bto prevent BGP from converging faster than IGP, which can cause temporary routing loops in the
Cto prevent routes which are learned both from IGP and BGP from forming a routing loop
Dto prevent churning between multiple available routes reachable through IGP and BGP

Explanation

Setting the IS-IS overload bit or advertising an infinite OSPF metric prevents other routers from forwarding transit traffic through a router whose IGP has converged but whose BGP table is not yet fully populated, avoiding black holes.

Common mistakes.

B. The problem being solved is the opposite - IGP converges faster than BGP, not the other way around, leaving a router with incomplete BGP forwarding information while IGP already routes transit traffic through it.
C. This describes an IGP-BGP redistribution loop, which is a separate problem addressed by route filtering or disabling BGP synchronization, not by metric manipulation or overload bits.
D. Route churning between IGP and BGP paths is addressed by BGP route dampening or timer tuning, not by advertising infinite IGP metrics or setting the IS-IS overload bit.

Concept tested. IGP and BGP convergence synchronization overload bit

Reference. https://www.rfc-editor.org/rfc/rfc3277

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