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)
