1.10 Troubleshoot OSPF (v2/v3)
1.10.c Network types, area types, and router types
📘CCNP Enterprise – ENARSI (300-410)
OSPF (Open Shortest Path First) needs to know how routers are connected to each other in a network because it affects neighbor discovery, hello intervals, and designated router (DR/BDR) election. This is defined by network types.
1. Point-to-Point (P2P)
- Definition:
A direct connection between exactly two routers. No other routers share the link. - Key Features:
- No need for DR/BDR election because only two routers exist.
- OSPF sends hellos directly to the neighbor using the interface IP.
- Default OSPF network type on a serial link is point-to-point.
- IT Example:
Two routers in different branches connected via a leased line or point-to-point VPN tunnel. Only these two routers talk OSPF to each other. - Exam Tips:
- DR/BDR election does not occur.
- OSPF over P2P automatically forms neighbor adjacency without extra configuration.
2. Broadcast
- Definition:
A network that supports broadcasting/multicasting (like Ethernet/LAN). Multiple routers can be connected on the same network segment. - Key Features:
- OSPF elects a DR (Designated Router) and BDR (Backup Designated Router) to reduce OSPF traffic.
- Routers use multicast hello messages (224.0.0.5) to discover neighbors.
- Default OSPF network type for Ethernet interfaces.
- IT Example:
A corporate LAN connecting multiple routers in the same building, all on the same VLAN, forming OSPF adjacencies. - Exam Tips:
- DR/BDR election is mandatory on broadcast networks.
- OSPF hello interval default is 10 seconds.
- Network must support multicast, otherwise neighbors won’t form automatically.
3. Nonbroadcast Multi-Access (NBMA)
- Definition:
A network that can connect multiple routers, but does not support broadcast/multicast. - Key Features:
- Needs manual neighbor configuration because automatic discovery does not work.
- DR/BDR election still required to reduce OSPF traffic.
- Often used on Frame Relay, X.25, or ATM networks.
- IT Example:
A Frame Relay WAN connecting several branch routers. Routers cannot send multicast hello, so each neighbor must be configured manually. - Exam Tips:
- Must manually configure neighbors.
- DR/BDR election reduces full LSDB exchange on large NBMA networks.
- Can simulate broadcast using OSPF point-to-multipoint settings.
4. Point-to-Multipoint (P2MP)
- Definition:
Combines characteristics of point-to-point and NBMA. OSPF treats the network as a collection of point-to-point links. - Key Features:
- No DR/BDR election.
- Works over nonbroadcast networks like Frame Relay.
- Supports multiple remote routers from a central router.
- Can be configured as point-to-multipoint or point-to-multipoint nonbroadcast.
- IT Example:
A hub-and-spoke WAN design using a central router connecting to multiple branch routers via virtual circuits. - Exam Tips:
- Treats each spoke as independent P2P connection.
- Simpler OSPF operation than NBMA because DR/BDR is not needed.
- Often used for VPNs or MPLS networks where direct point-to-point links exist logically.
Summary Table for Exam
| Network Type | DR/BDR Needed? | Neighbor Discovery | Example in IT Environment |
|---|---|---|---|
| Point-to-Point (P2P) | No | Auto, directly | Leased line / VPN tunnel |
| Broadcast | Yes | Auto (multicast) | LAN, VLANs in office building |
| Nonbroadcast (NBMA) | Yes | Manual | Frame Relay WAN |
| Point-to-Multipoint | No | Auto or manual | Hub-and-spoke WAN, MPLS VPN |
Important Exam Notes
- Default OSPF network type is based on interface type:
- Ethernet → Broadcast
- Serial → Point-to-Point
- DR/BDR election occurs only in multi-access networks (Broadcast and NBMA).
- Neighbor discovery:
- Auto discovery → P2P, Broadcast
- Manual configuration → NBMA
- Troubleshooting tips:
- If neighbors are not forming → check network type mismatch, hello/dead timers, DR/BDR status, or manual neighbor config.
- OSPF v3 (IPv6) behaves similarly regarding network types. Only multicast addresses change.
This explanation gives students both the concepts and how they are used in IT networks, without unnecessary analogies. They’ll be ready to identify, configure, and troubleshoot OSPF network types for the exam.
