3.6.d Implement and troubleshoot network types, area types and router types

3.6.d [i] Point-to-point, multipoint, broadcast, non-broadcast

The command used to set the network type of an OSPF interface is:

ip ospf network {broadcast | non-broadcast | point-to-multipoint}

Point-to-Point Sub-interfaces

A sub-interface is a logical way of defining an interface. The same physical interface can be split into multiple logical interfaces, with each sub-interface being defined as point-to-point. This was originally created in order to better handle issues caused by split horizon over NBMA and vector based routing protocols. A point-to-point sub-interface has the properties of any physical point-to-point interface. As far as OSPF is concerned, an adjacency is always formed over a point-to-point sub-interface with no DR or BDR election.

Point-to-Multipoint Interfaces

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. This concept takes the previously discussed point-to-point concept one step further. Administrators do not have to worry about having multiple subnets for each point-to-point link. The cloud is configured as one subnet. This should work well for people who are migrating into the point-to-point concept with no change in IP addressing on the cloud . Also, they would not have to worry about DRs and neighbor statements. OSPF point-to-multipoint works by exchanging additional link-state updates that contain a number of information elements that describe connectivity to the neighboring routers.

Broadcast Interfaces

In case of broadcast, the interface will be logically set to broadcast (using ip ospf network broadcast command) and will behave as if the router were connected to a LAN. DR and BDR election will still be performed so special care should be taken to assure either a full mesh topology or a static selection of the DR based on the interface priority.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3514-3523).  . Kindle Edition.

http://blog.ine.com/2007/12/29/understanding-the-ospf-point-to-multipoint-non-broadcast-network-type/