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| 4.6 |  | PPP and Layer 2 Considerations for Routed and Routing Protocols |
| 4.6.4 |  | IP split horizon checking |

Split horizon dictates that a routing update received on an interface cannot be retransmitted out of the same interface. This rule holds even if the routing update was received on one Frame Relay PVC and destined for retransmission onto another Frame Relay PVC. With reference to Figure , 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.

The concept of subinterfaces was originally created in order to handle issues caused by split horizon over nonbroadcast multiaccess (NBMA) networks and distance-vector-based routing protocols. Frame Relay and X.25 are examples of NMBA networks and IPX/SAP, RIP and Appletalk RTMP are examples of distance-vector-based routing protocols.

> Note: For TCP/IP, Cisco routers can disable split horizon on all Frame Relay interfaces and multipoint subinterfaces and, in fact, do this by default for most IP routing protocols. However, split horizon cannot be disabled for other routing protocols, such as those used with IPX and AppleTalk. These other protocols must use subinterfaces if dynamic routing is desired.

By dividing the partially meshed Frame Relay network into numerous virtual, point-to-point networks using subinterfaces, the split-horizon problem can be overcome. Each new point-to-point subnetwork is assigned its own network number. To the routed protocol, each subnetwork now appears to be located on a separate interface. Routing updates received from Site B on one logical point-to-point subinterface can be forwarded to Site C on a separate logical interface without violating split horizon.