I have read that when frame-relay is applied on hub-and-spoke topology - multicasts are possible and are more efficient in that network. but isn't frame relay is a NBMA network,which by definition cannot have broadcasts or multicasts?
Yes the default frame relay physical interface type is NBMA but you can override the default behavior by using the 'broadcast' keyword in frame relay map statements. This would cause broadcast/multicast packets to be sent over those DLCIs. This can particularly be useful for routing protocols to send hellos/updates using multicast packets.
Sundar approaches the question in one way and gives a good answer. I would like to approach the question differently in explaining this.
Multi access networks are broadcast/multicast capable or not capable. Broadcast/multicast capable multi access networks (for example Ethernet) are able to send a single physical frame on the media and that frame will reach all of the end stations on that media. For example on Ethernet a station sends an ARP request as a broadcast and that single frame is received and processed by every end station on that Ethernet.
Non broadcast multi access (NBMA) networks are not capable of sending a single frame and having that frame reach all end stations. As Sundar mentions the most common example of NBMA is Frame Relay. If a Frame Relay hub router has 6 PVCs (for example) it is not possible to transmit 1 frame and have it go over 6 PVCs. To compensate the router can emulate broadcast/multicast functionality by making copies of the broadcast or multi cast frame and sending a copy on each PVC. So routing protocols like EIGRP or OSPF that send routing updates as multicast can operate on Frame Relay multipoint if the Frame Relay map includes the broadcast keyword. If the map has broadcast specified the router will make a copy of the routing update and send a copy on each of the PVCs so that all the neighbors will receive it. But this is not true broadcast behavior because it was not a single frame.
It has been quite a while since I have done it and I do not currently have anything to test it with. I believe that Inverse ARP will insert the broadcast keyword in the map. Can you post the output of show frame-relay map?
I would also note that investigating Frame Relay multipoint is fine as a study exercise. In terms of practical application it is clear that the Best Practice for Frame Relay is point to point subinterface.
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