Split-horizon dictates that a routing update received on an interface cannot be retransmitted out onto the same interface. Referring to figure 2, this would mean that sites B and C can exchange routing information with site A, but would not be able to exchange routing information with each other. Split-horizon does not allow Site A to send routing updates received from Site B on to Site C and vice versa. However, split-horizon cannot be disabled for other protocols like IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired.
All nodes attached to a multipoint subinterface belong to the same network number.
Figure 4 shows serial 0.1 as a multipoint subinterface connecting to three different locations.