OSPF may behave differently depending on the type of interface involved. Here is a table with the default characteristics of OSPF as it pertains to specific network types.
When the following interface configurations are detected, they default to specific network types.
- Ethernet = Broadcast
- Serial (using PPP or HDLC encapsulation) = Point-to-Point
- Serial (using Frame-Relay) = NBMA
Generally, the following rules apply.
- Ethernet requires DR/BDR election and dynamic neighbor discovery.
- Nonbroadcast mediums do not support multicast and must be statically configured, and therefore do not support dynamic neighbor discovery.
- P2P mediums do not support DR/BDR elections because they either do not support more than 2 routers, or are assumed to be hub-spoke topologies.
The command to change the OSPF network type is ip ospf network <network_type>
Stuck-in-Active: Journal of an IT-Network Administrator 