OSPF Network Types Explained (Broadcast, Point-to-Point, NBMA)

Broadcast (Default on Ethernet)

Characteristics:

• DR/BDR election occurs
• Multicast Hellos (224.0.0.5)
• Assumes all routers can reach each other

Configuration:

interface gi0/0
 ip ospf network broadcast

Use when: Ethernet, all routers on same segment

Point-to-Point (Default on Serial)

Characteristics:

• No DR/BDR (only 2 routers)
• Multicast Hellos
• Fast adjacency

Configuration:

interface gi0/0
 ip ospf network point-to-point

Use when: Direct links, WAN point-to-point

Common use: Change Ethernet to P2P to avoid DR election

NBMA (Non-Broadcast Multi-Access)

Characteristics:

• DR/BDR election
• Unicast Hellos (no multicast)
• Requires neighbor statements

Configuration:

interface serial0/0
 ip ospf network non-broadcast
 ip ospf priority 100

router ospf 1
 neighbor 10.1.1.2

Use when: Frame Relay, X.25

Note: Rarely used in modern networks

Point-to-Multipoint

Characteristics:

• No DR/BDR
• Multicast Hellos
• Treats as collection of P2P links

Configuration:

interface serial0/0
 ip ospf network point-to-multipoint

Use when: MPLS, Metro Ethernet partial mesh

Changing Network Type

Check current type:

R1# show ip ospf interface gi0/0 | include Network Type
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST

Change to point-to-point:

interface gi0/0
 ip ospf network point-to-point

Verify:

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2         0     FULL/  -        00:00:35    10.1.1.2        Gi0/0

Note: FULL/ - (no DR/BDR on P2P)

Summary

✅ Broadcast — Ethernet default, DR/BDR
✅ Point-to-Point — Serial default, no DR/BDR
✅ NBMA — Frame Relay, unicast Hellos
✅ Point-to-Multipoint — MPLS/Metro, no DR/BDR
✅ Change with ip ospf network command

Next: Troubleshooting Area Mismatch (Article 19)

Internal Links:

• ← OSPF Timers (Article 16)
• ← OSPF DR/BDR (Article 5)
• → OSPF Neighbors Not Forming (Article 18)