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352-001 · Question #399

You are redesigning an OSPF v2 network and must migrate some links. You are concerned that there are different subnet masks. Which link type will still form an OSPF adjacency even if there are subnet

The correct answer is D. point-to-point. OSPF point-to-point links skip subnet mask verification during neighbor adjacency formation, making them the only link type that tolerates mismatched masks.

Layer 3 Control Plane

Question

You are redesigning an OSPF v2 network and must migrate some links. You are concerned that there are different subnet masks. Which link type will still form an OSPF adjacency even if there are subnet mask mismatches?

Options

  • Abroadcast
  • Bpoint-to-multipoint
  • Cnon-broadcast
  • Dpoint-to-point

How the community answered

(35 responses)
  • A
    3% (1)
  • B
    6% (2)
  • D
    91% (32)

Why each option

OSPF point-to-point links skip subnet mask verification during neighbor adjacency formation, making them the only link type that tolerates mismatched masks.

Abroadcast

Broadcast network types require matching subnet masks in OSPF hello packets, and a mismatch will prevent adjacency formation.

Bpoint-to-multipoint

Point-to-multipoint networks also validate subnet masks during the neighbor discovery process, so a mismatch will block adjacency.

Cnon-broadcast

Non-broadcast (NBMA) networks require matching subnet masks just like broadcast types, and mismatches will cause neighbor formation to fail.

Dpoint-to-pointCorrect

On point-to-point OSPF links, the subnet mask is not checked as part of the hello packet validation process, so neighbors will still form an adjacency even with mismatched masks. All other OSPF network types (broadcast, non-broadcast, point-to-multipoint) require matching subnet masks because they use the mask to verify that neighbors belong to the same network segment. This behavior makes point-to-point the correct choice when migrating links with inconsistent subnet configurations.

Concept tested: OSPF network types and subnet mask adjacency rules

Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

Topics

#OSPFv2#subnet mask mismatch#adjacency#point-to-point links

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