IP split horizon checking is disabled by default for Frame Relay encapsulation to allow routing updates to go in and out of the same interface. An exception is EIGRP for which split horizon must be explicitly disabled.

Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows you to overcome split-horizon rules so packets received on one virtual interface can be forwarded to another virtual interface, even if they are configured on the same physical interface.