Understand Open Shortest Path First (ospf) Design Guide

neighbor ip-address [priority number] [poll-interval seconds]
command, 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. The "poll-interval" is the amount
of time an NBMA interface waits before the poll (a sent Hello) to a presumably dead neighbor.
The 
neighbor
command applies to routers with DR- or BDR-potential (interface priority not equal to
0). This shows a network diagram where DR selection is very important:

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In this diagram, it is essential for the RTA interface to the cloud to be elected DR. This is because
RTA is the only router that has full connectivity to other routers.
The election of the DR could be influenced by the ospf priority parameter on the interfaces.
Routers that do not need to become DRs or BDRs have a priority of 0 other routers could have a
lower priority.
The
neighbor
command is not covered in depth in this document and becomes obsolete through
new interface Network Type irrespective of the underlying physical media. This is explained in the
next section.
Avoid DRs and neighbor Command on NBMA
Different methods can be used to avoid the complications of static neighbor configuration and
specific routers which become DRs or BDRs on the non-broadcast cloud.
To specify which method to use is influenced by whether we are start the network from the start or
if we rectify a design which already exists.
Point-to-Point Subinterfaces
A subinterface is a logical way to define an interface. The same physical interface can be split into
multiple logical interfaces, with each subinterface 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 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.
This is an illustration of point-to-point subinterfaces:

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In this diagram, on RTA, we can split Serial 0 into two point-to-point subinterfaces, S0.1 and S0.2.
This way, OSPF considers the cloud as a set of point-to-point links rather than one multi-access
network.
The only drawback for the point-to-point is that each segment belongs to a different subnet. This
is unacceptable because some administrators have already assigned one IP subnet for the whole
cloud.
Another workaround is to use IP unnumbered interfaces on the cloud. This is also a problem for
administrators who manage the WAN based on IP addresses of the serial lines. This is a typical
configuration for RTA and RTB:
RTA#
interface Serial 0
no ip address
encapsulation frame-relay
interface Serial0.1 point-to-point
ip address 198.51.100.36 255.255.252.0
frame-relay interface-dlci 20
interface Serial0.2 point-to-point
ip address 198.51.100.46 255.255.252.0
frame-relay interface-dlci 30
router ospf 10
network 198.51.100.1 0.0.255.255 area 1
RTB#
interface Serial 0
no ip address

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 encapsulation frame-relay
interface Serial0.1 point-to-point
ip address 198.51.100.35 255.255.252.0
frame-relay interface-dlci 40
interface Serial1
ip address 198.51.100.11 255.255.255.0
router ospf 10
network 198.51.100.1 0.0.255.255 area 1
network 198.51.100.10 0.0.255.255 area 0