hardMultiple Choice
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
Serial interfaces with HDLC encapsulation default to point-to-point OSPF network type, not broadcast. Broadcast is tempting because Ethernet interfaces use it by default, but HDLC serial links have no MAC layer or DR/BDR election, so OSPF treats them as point-to-point.
- ✗
Non-broadcast (NBMA)
Why it's wrong here
HDLC serial interfaces default to point-to-point OSPF network type, not NBMA. NBMA is tempting because Frame Relay and X.25 multipoint interfaces use it, requiring neighbour statements, but HDLC provides a single point-to-point link with no address mapping.
- ✓
Point-to-point
Why this is correct
HDLC serial links default to the point-to-point OSPF network type, which suppresses DR/BDR election and sends multicast hellos every 10 seconds. This satisfies the stem's HDLC encapsulation constraint, since Cisco assigns point-to-point automatically to serial interfaces rather than broadcast, non-broadcast or point-to-multipoint.
- ✗
Point-to-multipoint
Why it's wrong here
HDLC serial interfaces default to point-to-point OSPF network type, not point-to-multipoint. Point-to-multipoint is tempting because it is a valid OSPF network type used on Frame Relay or NBMA multipoint interfaces, where a single subnet spans several neighbours without full mesh adjacency.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path 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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