It may sound very basic, But I am not find the convincing answer in the Internet. Finally i sought your help
Let us start the argument with the below points
1. OSPF & EIGRP both use multicast address to find the neighbours and form adjacencies.
2. NBMA network donot support Broadcast / Multicast natively.
If this is the case,
In NBMA Network like Frame Relay, why OSPF is alone having multiple options like NON-BROADCAST, POINT-TO-POINT, BROADCAST,POINT-TO-MULTIPOINT but where as EIGRP donot have these confusion and straight away forms the neighbour relation.
show ip ospf interface
Serial0/0 is up, line protocol is up
Internet Address 22.214.171.124/24, Area 4
Process ID 1, Router ID 126.96.36.199, Network Type NON_BROADCAST, Cost: 64
Another Interesting point is I have configured an Interface with encapsulation framrelay and OSPF is configured in the network and in the output of "show int ser 0/0" command I am seeing Network Type as "NON_BROADCAST". But when i ping 188.8.131.52 I am getting reply from the peer routers as
Reply to request 0 from 184.108.40.206, 8 ms
I expected that reply will not come as the interface is a NON_BROADCAST.
Can you help me what is the actual reasons for OSPF network types and why I am able to ping the Multicast IP even though the interface is NON_BROADCAST
Thanks in advance
For the problem with multiaccess network with a partial neighbor reachability I have just described, it is actually more beneficial to use the point-to-multipoint approach and to advertise a link between each pair of routers on a multiaccess segment if they can successfully hear their Hello packets. No "pseudonode" representing the network fill be advertised (if it were, we would not be able to say in OSPF that the spokes are not able to communicate directly). The memory requirements are increased but the topology table now accurately reflects the actual reachability of neighbors within the network.
So much to the first aspect - to the modelling of the network in the topological database. The second point was related to the multicast/broadcast capabilities of the underlying Layer2 technology.
One of the major functions of routing protocols is to dynamically detect present and future neighbors if possible, without apriori knowing their address. OSPF uses the multicast address 220.127.116.11 to send and listen for Hello packets to dynamically discover neighbors. For an OSPF router, it simply means sending an IP packet to the destination address 18.104.22.168 and it is up to the network technology to replicate that multicast. We call these networks broadcast networks. The multicasts are also used to send routing information to multiple neighbors at once.
Now, again, this works in a LAN environment such as Ethernet. However, virtual-circuit based technologies such as ATM, X.25 or Frame Relay are not natively capable of replicating a multicast/broadcast and sending it automatically through a number of VCs. That is why we call them non-broadcast networks. In these networks, you do not have the luxury of automatically discovering OSPF neighbors, and instead, you have to preconfigure their addresses manually. The communication between such neighbors will be unicast-only.
Now, routers are actually able to cheat a little here. Even though a Frame Relay is a non-broadcast technology, the IP/DLCI mappings may have the 'broadcast' flag set that simply tells the router to send a copy of each outgoing multicast/broadcast packet through that DLCI. For the Frame Relay, it is a packet like all others, however, your router has done the hard work of replicating the multicast on all PVCs.
Now, when you combine these two aspects, you will come to a number of combinations:
1.) Broadcast MultiAccess (BMA) - this is the usual type for Ethernet. Here, the routers are discovered automatically using multicasts and the network itself is modelled as a pseudonode, decreasing the memory footprint
2.) Non-Broadcast Multi-Access (NBMA) - this is the default network type for Frame Relay interfaces. The neighbors must be defined statically (OSPF will not even try to send a multicast packet out an interface of this network type!), however, the network will be modelled using a pseudonode. Actually, this network type is not so appropriate for most Frame Relay networks because in most cases, they are not fully meshed and the problem with such deployments has been discussed in my earlier post. Even an intermittent or inadvertent failure of a single PVC in a Frame Relay network that was originally fully meshed will make this network partial-meshed with all negative impacts.
3.) Point-to-Point - this is the default for Frame-Relay point-to-point subinterfaces. The neighbor is discovered automatically and the link between the two routers on the PVC is modelled as a direct connection of those two routers. No pseudonode is used. The 'broadcast' has no meaning here because of the point-to-point nature.
4.) Point-to-Multipoint Broadcast - this network type must be configured manually. Neighbors will be discovered automatically (using the 'broadcast' flag in IP/DLCI mappings on Frame Relay) and the connections will be modelled by direct links. No pseudonode will be used.