8.3 Frame Relay Topologies
8.3.2 Reachability issues with routing updates
By default, a Frame Relay network provides NBMA connectivity between remote sites. NBMA connectivity means that although all locations can reach each other, depending on the topology, routing update broadcasts received by one router cannot be forwarded to all locations because Frame Relay networks use split horizon to reduce the number of routing loops.

Split horizon reduces the number of routing loops by not allowing a routing update received on one interface to be forwarded through the same interface. As shown in the Figure, central router interface S0 receives a routing update from router Branch A. Central router is connecting three PVCs over a single interface. Split horizon forbids the central router to send out updates via the same interface that it received them. Therefore, Branch B and Branch C routers never receive the update.

There are two inherent problems with multiple PVCs terminating in one interface: split horizon and the support for broadcast traffic.

Broadcasts are not a problem if there is only a single PVC on a physical interface because this would be more of a point-to-point connection type.

Another issue with routers that support multipoint connections over a single interface is that when many DLCIs terminate in a single router, that router must replicate routing updates and service advertising updates on each DLCI to the remote routers. The updates can consume access-link bandwidth and cause significant latency variations in user traffic. The updates can also consume interface buffers and lead to higher packet-rate loss for both user data and routing updates.

The amount of broadcast traffic and the number of VCs terminating at each router should be evaluated during the design phase of a Frame Relay network. Overhead traffic, such as routing updates, can affect the delivery of critical user data, especially when the delivery path contains low-bandwidth (56-Kbps) links.

One common solution to split horizon is subinterfaces, which are covered in the upcoming section.