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352-001 · Question #45

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?

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.

Layer 3 Control Plane

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

How the community answered

(30 responses)
  • A
    63% (19)
  • B
    10% (3)
  • C
    7% (2)
  • D
    20% (6)

Why each option

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.

Ato prevent traffic loss when the path from a router to a BGP next hop traverses another router thatCorrect

When a router restarts, IGP reconverges quickly and peers begin sending traffic through it, but if BGP has not yet fully loaded its routing table, the router will drop packets destined for BGP-learned prefixes that transit it. Advertising an infinite OSPF metric or setting the IS-IS overload bit signals other routers to avoid using it as a transit node until BGP convergence is complete, eliminating this black-hole condition.

Bto prevent BGP from converging faster than IGP, which can cause temporary routing loops in the

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.

Cto prevent routes which are learned both from IGP and BGP from forming a routing loop

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.

Dto prevent churning between multiple available routes reachable through IGP and BGP

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

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

Topics

#BGP-IGP convergence#IS-IS overload bit#OSPF max metric#traffic blackholing prevention

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