http://www.cisco.com/en/US/docs/ios/12_0/np1/configuration/guide/1cospf.html#wp4739

Configure Your OSPF Network Type

You have the choice of configuring your OSPF network type as either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. This feature saves you from having to configure neighbors, as described in the section “Configure OSPF for Nonbroadcast Networks.”

Configuring nonbroadcast, multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully meshed network. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers not directly connected will go through the router that has virtual circuits to both routers. Note that it is not necessary to configure neighbors when using this feature.

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to nonbroadcast multiaccess and point-to-point networks:

•Point-to-multipoint is easier to configure because it requires no configuration of neighbor commands, it consumes only one IP subnet, and it requires no designated router election.

•It costs less because it does not require a fully meshed topology.

•It is more reliable because it maintains connectivity in the event of virtual circuit failure.

To configure your OSPF network type, use the following command in interface configuration mode:

See the “OSPF Point-to-Multipoint Example” section at the end of this chapter for an example of an OSPF point-to-multipoint network.

Configure Point-to-Multipoint, Broadcast Networks

On point-to-multipoint, broadcast networks, there is no need to specify neighbors. However, you can specify neighbors with the neighbor command, in which case you should specify a cost to that neighbor.

Before this feature, some OSPF point-to-multipoint protocol traffic was treated as multicast traffic. Therefore, the neighbor command was not needed for point-to-multipoint interfaces because multicast took care of the traffic. Hellos, updates and acknowledgments were sent using multicast. In particular, multicast hellos discovered all neighbors dynamically.

On any point-to-multipoint interface (broadcast or not), the Cisco IOS software assumed the cost to each neighbor was equal. The cost was configured with the ip ospf cost command. In reality, the bandwidth to each neighbor is different, so the cost should be different. With this feature, you can configure a separate cost to each neighbor. This feature applies to point-to-multipoint interfaces only.

To treat an interface as point-to-multipoint broadcast and assign a cost to each neighbor, use the following commands beginning in interface configuration mode:

Configure OSPF for Nonbroadcast Networks

Because there might be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.

These parameters need only be configured in those devices that are themselves eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).

To configure routers that interconnect to nonbroadcast networks, use the following command in router configuration mode:

You can specify the following neighbor parameters, as required:

•Priority for a neighboring router

•Nonbroadcast poll interval

•Interface through which the neighbor is reachable

On point-to-multipoint, nonbroadcast networks, you now use the neighbor command to identify neighbors. Assigning a cost to a neighbor is optional.

Prior to Release 12.0, some customers were using point-to-multipoint on nonbroadcast media (such as classic IP over ATM), so their routers could not dynamically discover their neighbors. This feature allows the neighbor command to be used on point-to-multipoint interfaces.

On any point-to-multipoint interface (broadcast or not), the Cisco IOS software assumed the cost to each neighbor was equal. The cost was configured with the ip ospf cost command. In reality, the bandwidth to each neighbor is different, so the cost should be different. With this feature, you can configure a separate cost to each neighbor. This feature applies to point-to-multipoint interfaces only.

To treat the interface as point-to-multipoint when the media does not support broadcast, use the following commands beginning in interface configuration mode:

See–>  http://packetlife.net/static/cheatsheets/ospf.pdf
NETWORK TYPES

Nonbroadcast     –   (NBMA)

DR/BDR  Eelected               YES
Neighbor Discovery            NO
Hello/Dead Timers             30/120
Standard                             RFC 2328
Supported Technology        FULL MESH

Multipoint   – Broadcast

DR/BDR  Eelected            NO
Neighbor Discovery        YES
Hello/Dead Timers         30/120
Standard                         RFC 2328
Supported Technology    ANY
Multipoint – Nonbroadcast

DR/BDR  Eelected             NO
Neighbor Discovery         NO
Hello/Dead Timers          30/120
Standard                          CISCO
Supported Technology    ANY

Broadcast

DR/BDR  Eelected             YES
Neighbor Discovery         YES
Hello/Dead Timers          10/1940
Standard                          CISCO
Supported Technology    FULL MESH

Point-to-Point

DR/BDR  Eelected                NO
Neighbor Discovery            YES – multicast hello’s
Hello/Dead Timers             10/1940
Standard                             CISCO
Supported Technology        POINT-TO-POINT

sh ip ospf ?
<1-65535>            Process ID number
border-routers       Border and Boundary Router Information
database             Database summary
flood-list           Link state flood list
interface            Interface information
max-metric           Max-metric origination information
mpls                 MPLS related information
neighbor             Neighbor list
request-list         Link state request list
retransmission-list  Link state retransmission list
sham-links           Sham link information
statistics           Various OSPF Statistics
summary-address      Summary-address redistribution Information
timers               OSPF timers information
traffic              Traffic related statistics
virtual-links        Virtual link information
|                    Output modifiers

debug ip ospf ?
adj             OSPF adjacency events
database-timer  OSPF database timer
events          OSPF events
flood           OSPF flooding
hello           OSPF hello events
lsa-generation  OSPF lsa generation
mpls            OSPF MPLS
nsf             OSPF non-stop forwarding events
packet          OSPF packets
retransmission  OSPF retransmission events
spf             OSPF spf
tree            OSPF database tree

sh ip ospf int brief

1. the network statement under the routing process.
2. ip ospf [process-id] area [area-id] at the interface level.