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352-001 · Question #299

Which statement about the behavior of OSPF on a hub-and-spoke topology is true?

The correct answer is C. The DR election is a challenge unless a point-to-point network type is used.. In hub-and-spoke topologies, the DR election is problematic because spokes cannot communicate directly, making the hub the mandatory DR - a condition that requires careful configuration unless point-to-point network types are used.

Layer 3 Control Plane

Question

Which statement about the behavior of OSPF on a hub-and-spoke topology is true?

Options

  • AAdditional host routes are added to the routing table on a NBMA network type.
  • BThe DR and BDR election occurs regardless of the underlying OSPF network type.
  • CThe DR election is a challenge unless a point-to-point network type is used.
  • DTraffic does not need to traverse the hub to reach the spokes.

How the community answered

(31 responses)
  • B
    6% (2)
  • C
    90% (28)
  • D
    3% (1)

Why each option

In hub-and-spoke topologies, the DR election is problematic because spokes cannot communicate directly, making the hub the mandatory DR - a condition that requires careful configuration unless point-to-point network types are used.

AAdditional host routes are added to the routing table on a NBMA network type.

OSPF does not add additional host routes for NBMA network types; host route injection is not a standard OSPF NBMA behavior.

BThe DR and BDR election occurs regardless of the underlying OSPF network type.

DR and BDR elections do not occur on all OSPF network types - point-to-point and point-to-multipoint network types specifically skip the DR/BDR election process.

CThe DR election is a challenge unless a point-to-point network type is used.Correct

On a hub-and-spoke NBMA topology, all spoke routers connect only to the hub, meaning the hub must win the DR election to ensure proper LSA flooding to all spokes. If a spoke becomes DR, other spokes cannot receive updates directly from it, breaking adjacency. Configuring point-to-point (or point-to-multipoint) network types eliminates the DR/BDR election entirely, resolving this challenge.

DTraffic does not need to traverse the hub to reach the spokes.

In a hub-and-spoke topology, all traffic between spokes must traverse the hub because spokes have no direct connectivity to each other.

Concept tested: OSPF DR election behavior on hub-and-spoke NBMA

Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-ospf-hub-spoke.html

Topics

#OSPF hub-and-spoke#DR election#NBMA network type#point-to-point

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