OSPF Non-Broadcast Multiple-Access (NBMA) Networks

On the Multipoint Frame-Relay network the default OSPF type is NON-BROADCAST. This means that the OSPF Neighbors will not be formed like on the standard Broadcast Network Segment.

 Cisqueros_R1#show ip ospf inter s1/0
 Serial1/0 is up, line protocol is up 
   Internet Address 10.1.1.1/24, Area 0 
   Process ID 1, Router ID 1.1.1.1, Network Type NON_BROADCAST, Cost: 64
   Topology-MTID    Cost    Disabled    Shutdown      Topology Name
         0           64        no          no            Base
        ...
        
So in order to establish the OSPF Neighbors, we can for example use the "network" command in order to transform the OSPF link from MULTICAST to UNICAST

 (config-router)#neighbor 172.128.185.66

*In HUB-AND-SPOKE the Spokes do not have the Layer 2 reachability, so this command makes no sense. Instead just be sure to set their OSPF priority to 0, so that they dont participate the DR/BDR Election

 (config-if)#ip ospf priority 0

 The HUB Router will be elected as DR on every Link and exchange OSPF Database with each of the Spokes:

 Cisqueros_R1#show ip ospf neighbor <--- R1 IS THE HUB

 Neighbor ID     Pri   State           Dead Time   Address         Interface
 2.2.2.2           0   FULL/DROTHER    00:01:51    10.1.1.2        Serial1/0
 3.3.3.3           0   FULL/DROTHER    00:01:51    10.1.1.3        Serial1/0
 4.4.4.4           0   FULL/DROTHER    00:01:56    10.1.1.4        Serial1/0

*In this kind of OSPF Topology - it's not necessary to have the Frame-Relay interface configured with the "broadcast" keyword, because we are manually defining the OSPF Neighbor and turning the Links into UNICASTS.


OSPF BROADCAST Networks v.s. POINT-TO-POINT Networks

If you wish to convert the previous network into the Broadcast Network, the following command needs to be applied:

 (config-if)#ip ospf network broadcast

And in case it needs to be Point-to-Point:

 (config-if)#ip ospf network point-to-point

The main difference here is the NEXT HOP:
 BROADCAST: Next Hop is the router that ORIGINATED the Route
 POINT-TO-POINT: Next Hop is the router that ADVERTISED the Route