Network types, DR and BDR
How OSPF behaves on an interface depends on its network type:
| Network type | Typical use | DR/BDR | Hello / Dead |
|---|---|---|---|
| broadcast | Ethernet default | Yes | 10 / 40 |
| point-to-point | Router-to-router links (configure explicitly on Ethernet) | No | 10 / 40 |
| non-broadcast (NBMA) | Legacy Frame Relay | Yes, neighbours configured | 30 / 120 |
| point-to-multipoint | Hub-and-spoke | No | 30 / 120 |
On a multi-access LAN, a full mesh of adjacencies would mean every change is flooded once per pair of routers. OSPF elects a Designated Router (DR) and Backup DR (BDR) to act as the hub.
Ethernet defaults to broadcast, even when there are only two routers on the link. That means a pointless DR election, an extra Type 2 LSA, and slower adjacency formation (the router waits up to the dead interval to elect). On router-to-router links, configure ip ospf network point-to-point: it's the most common OSPF tuning in production. Both ends must match.
- Everyone says Hello. All four routers send Hellos to 224.0.0.5. Every router hears every other — without a DR, each would form a full adjacency with all three others: n(n−1)/2 = 6 adjacencies, and every change flooded six ways.
- Election. Highest priority wins (tie: highest router ID). R4 becomes the Designated Router and R3 the Backup. Priority 0 means "never DR". The election is not pre-emptive: a better router joining later does not take over.
- Adjacencies only with DR and BDR. R1 and R2 go Full with R4 (DR) and R3 (BDR) only. Between themselves they stay at 2-Way — this is the expected state, not a fault.
- A change is sent to the DR. R1's link changes. Instead of flooding to everyone, it sends the LSU to 224.0.0.6 (AllDRouters), which only the DR and BDR listen to.
- The DR re-floods once. The DR sends the update to 224.0.0.5 so every router gets it in one transmission. The BDR stays silent unless the DR stops acknowledging.
- The DR speaks for the segment. The DR also originates a Network LSA (Type 2) listing every router attached to the LAN. In the SPF graph the segment becomes a pseudo-node that all four routers connect to.
Check your understanding
On a LAN with five OSPF routers, R1 (DROther) shows R2 (also DROther) in 2-Way. What should you do?
- Nothing — this is expected
- Fix the MTU mismatch
- Raise R1's priority so it becomes DR
- Change the network type to point-to-point