By default, a Frame Relay network provides an NBMA connectivity between remote sites. An NBMA environment is treated like other broadcast media environments, such as Ethernet, where all the routers are on the same subnet. However, to reduce cost, NBMA clouds are usually built in a hub-and-spoke topology. With a hub-and-spoke topology, the physical topology does not provide the multi-access capabilities that Ethernet does, so each router may not have separate PVCs to reach the other remote routers on the same subnet.
Two problems that the Frame Relay NBMA topology may cause are reachability issues regarding routing updates and the need to replicate broadcasts onto each PVC when a physical interface contains more than one PVC, as follows:
Routing update reachability: Split horizon updates reduce routing loops by preventing a routing update received on an interface to be forwarded out the same interface. In a scenario using a hub-and-spoke Frame Relay topology, a remote router (a spoke router) sends an update to the headquarters router (the hub router) that is connecting multiple PVCs over a single physical interface. The headquarters router then receives the broadcast on its physical interface but cannot forward that routing update through the same interface to other remote (spoke) routers. Split horizon is not a problem if there is only a single PVC on a physical interface because this type of connection would be more of a point-to-point connection type.
Broadcast replication: With routers that support multipoint connections over a single interface, terminating many PVCs, the router must replicate broadcast packets (like routing update broadcasts) on each PVC to the remote routers. These replicated broadcast packets consume bandwidth and cause significant latency variations in user traffic.