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352-001 · Question #238

The IGP next-hop reachability for a BGP route is lost but a default route is available. Assuming that BGP connectivity is maintained, what will happen to the BGP route?

The correct answer is C. It will be considered a valid route. When a BGP route's IGP next-hop is unreachable but a default route exists, BGP performs a recursive lookup and resolves the next-hop via the default route, keeping the route valid.

Layer 3 Control Plane

Question

The IGP next-hop reachability for a BGP route is lost but a default route is available. Assuming that BGP connectivity is maintained, what will happen to the BGP route?

Options

  • AIt will be removed from the BGP table.
  • BIt will be considered invalid for traffic forwarding.
  • CIt will be considered a valid route.
  • DIt will be put in a hold-down state by BGP until the next hop has been updated.

How the community answered

(32 responses)
  • A
    3% (1)
  • B
    6% (2)
  • C
    78% (25)
  • D
    13% (4)

Why each option

When a BGP route's IGP next-hop is unreachable but a default route exists, BGP performs a recursive lookup and resolves the next-hop via the default route, keeping the route valid.

AIt will be removed from the BGP table.

BGP does not remove a route from its table solely because the IGP next-hop is unreachable - the route stays in the BGP table and a default route prevents it from being marked invalid.

BIt will be considered invalid for traffic forwarding.

The route is not marked invalid for forwarding because the default route satisfies the recursive next-hop lookup, making the next-hop effectively reachable.

CIt will be considered a valid route.Correct

BGP performs a recursive next-hop lookup in the IP routing table before marking a route valid or invalid. If the specific IGP next-hop prefix is gone but a default route (0.0.0.0/0) is present, the recursive lookup succeeds via the default, so BGP considers the next-hop reachable and the route remains valid for installation in the RIB.

DIt will be put in a hold-down state by BGP until the next hop has been updated.

BGP has no hold-down mechanism for routes with unresolvable next-hops; it simply marks them best or not-best based on the recursive lookup result.

Concept tested: BGP recursive next-hop lookup using default route

Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Topics

#BGP#next-hop reachability#default route#route validity

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