OSPF routers discover neighbors by exchanging Hello packets. On broadcast networks, Hellos go to multicast address (AllSPFRouters).
Hellos include:
- Router ID
- Area ID
- Hello and Dead intervals
- Network mask
- Authentication data
For neighbors to form, these must match:
- Area ID
- Hello and Dead intervals
- Authentication
- Subnet mask (on broadcast networks)
By default, Hellos send every seconds on broadcast networks. If no Hello arrives for seconds (Dead interval), the neighbor is declared down.