here is what the documentation says :
point-to-point
--------------------
A point-to-point subinterface 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 subinterface with no DR or BDR election.
point-to-multipoint
--------------------------
An OSPF point-to-multipoint interface is defined as a numbered point-to-point
interface having one or more neighbors. This concept takes the pre-viously
discussed point-to-point concept one step further. Administra-tors
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 infor-mation
elements that describe connectivity to the neighboring routers.
non-broadcast
--------------------
The selection of the DR becomes
an issue because the DR and BDR need to have full physical connectivity
with all routers that exist on the cloud. Also, because of the lack of
broadcast capabilities, the DR and BDR need to have a static list of all
other routers attached to the cloud. This is achieved using the neighbor
command:
neighbor ip-address [priority number] [poll-interval seconds]
where the “ip-address” and “priority” are the IP address and the OSPF
priority given to the neighbor. A neighbor with priority 0 is considered
ineligible for DR election.
broadcast
---------------
This approach is a workaround for using the “neighbor” command which
statically lists all existing neighbors. The interface will be logically
set to broadcast 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.
