OSPF network types: non-broadcast
OSPF network types: multicast without broadcast
When you build a routed network using OSPF as your routing protocol of choice, you should be aware of the fact that OSPF will automatically detect the Layer 2 encapsulation type and adopt one of 6 ways to operate on that link.
The first of these 6 behaviors that we will cover is OSPF Non-Broadcast mode.
This non-broadcast mode is the default state in which OSPF will establish an interface when executing the “frame relay encapsulation” command on a serial interface.
OSPF will treat this medium like any other transmission medium such as Ethernet.
NBMA clouds are generally built in a hub and spoke topology. PVCs are arranged in a partial mesh and the physical topology does not provide the multiple access that OSPF believes exists.
OSPF in this mode will also try to choose a designated router and a subsequent designated router. Selecting the DR and BDR can become a problem because the DR and BDR must have full physical connectivity to all other OSPF routers that exist on the non-streaming network. If you have a network of hubs and radios where a router has Frame Relay PVCs for each radio, then logically you would configure the Hub to be the DR and the radios not to consider themselves part of the election process. To do this you must configure the radios with the following command: (Assuming that we are using the physical interface)
serial interface 0
ip ospf priority 0
On the Hub router to be on the safe side, set the following command to become the DR. (Assuming we are using the physical interface)
serial interface 0
ip ospf priority 255
OSPF will not send any multicast traffic over non-broadcast media and because of this you will need to configure the DR with (and BDR if you have one) a static list of all other routers connected to the non-broadcast network, in The In the example below, our neighbors are 10.1.1.2 and 10.1.1.3.
To do this, you will need to use the following command:
ospf router 1
neighbor 10.1.1.2 10
neighbor 10.1.1.3 10
The “poll interval” is the amount of time an NBMA interface waits before polling (sending a hello) to a suspected dead neighbor. The neighbor command applies to routers with the potential to be DR or BDR (interface priority is not equal to 0).
Additionally, OSPF will change the interface timers to 30 seconds Hello and 120 Dead Timer.