Split horizon is the loop avoidance mechanism that drives the requirement to create sub interfaces for each point-to-point connection in a partially meshed frame relay network. Frame relay is a non-broadcast multi-access (NBMA) network and obeys the rules of split horizon. This mechanism prohibits a routing protocol from sending updates out the same physical interface on which it was received. When the same physical interface is used to host multiple frame relay connections, this will prevent an update arriving from remote network A on the physical interface from being sent out the same interface to remote network B.

By creating a subinterface for each frame relay connection and assigning IP addresses to the subinterfaces rather than the physical interface, and by placing the subinterfaces into different subnets, split horizon will not see the "virtual" interfaces as the same interface and will allow these routing updates to be sent back out the same physical interface on which they arrived. It is important to map each subnet (or subinterface) to a remote Data Link Connection Identifier (DLCI) so that traffic to a remote network can be sent out the correct subinterface.

To summarize this discussion:
* Subinterfaces solve the NBMA split horizon issues.
* There should be one IP subnet mapped to each DLCI

Poison reverse is not the mechanism driving the requirement to create subinterfaces for each point-to-point connection in a partially meshed frame relay network. This mechanism requires a router to send an unreachable metric to the interface on which a network was discovered when it is learned from another interface that the network is no longer available.

Maximum hop count is not the mechanism driving the requirement to create sub interfaces for each point-to-point connection in a partially meshed frame relay network. Each routing protocol has a maximum hop count, which is the maximum number of hops allowed to a remote network before the network is considered "unreachable".

Feasible successor is not the mechanism driving the requirement to create sub interfaces for each point-to-point connection in a partially meshed frame relay network. This is a concept unique to EIGRP that represents a secondary route to a network that is considered the "best" route of possible backup routes.