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300-410 Practice Question: The default OSPF network type for a serial…

What is the default OSPF network type for a serial interface configured with HDLC encapsulation on Cisco routers?

⚠ Common exam trap

Many candidates confuse the default OSPF network type for serial interfaces with the default for Ethernet (broadcast) or Frame Relay (NBMA), forgetting that HDLC encapsulation forces a point-to-point OSPF network type.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

Point-to-point

On Cisco routers, a serial interface using HDLC encapsulation defaults to the OSPF network type point-to-point. This is because HDLC is a synchronous framing protocol that inherently implies a direct, single-neighbor link, so OSPF automatically sets the network type to point-to-point, which requires no DR/BDR election and uses multicast Hello packets (224.0.0.5).

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

  • Broadcast

    Why it's wrong here

    Broadcast is the default for Ethernet interfaces, not serial HDLC.

  • Non-broadcast (NBMA)

    Why it's wrong here

    NBMA is the default for Frame Relay multipoint interfaces, not HDLC.

  • Point-to-point

    Why this is correct

    Correct: Serial HDLC defaults to point-to-point, enabling faster convergence without DR/BDR election.

  • Point-to-multipoint

    Why it's wrong here

    Point-to-multipoint is not a default network type for any interface; it must be configured manually.

Visual reference

R1 R2 R3 R4 10 100 10 100 OSPF picks R1→R2→R4 (cost 20) over R1→R3→R4 (cost 200)

Quick reference

Routing Protocol Comparison

ProtocolMetricMax HopsAlgorithmType
RIP v2Hop count15Bellman-FordDistance vector
OSPFCost (bandwidth)UnlimitedDijkstra (SPF)Link state
EIGRPComposite metricUnlimitedDUALHybrid
IS-ISCostUnlimitedDijkstraLink state
BGPPolicy / attributesUnlimitedPath vectorPath vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

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Written by Johnson Ajibi, MSc IT Security

Senior Network & Security Engineer · founder of Courseiva

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