In any Frame Relay topology, when a single interface must be used to interconnect multiple
sites, you can have reachability issues because of the NBMA nature of Frame Relay. The
Frame Relay NBMA topology can cause the following two problems:
?– Routing update reachability: Split horizon updates reduce routing loops by
preventing a routing update that is received on an interface from being forwarded out
the same interface. In a scenario using a hub-and-spoke Frame Relay topology, a
remote router (a spoke router) sends an update to the headquarters router (the hub
router) that is connecting multiple PVCs over a single physical interface. The
headquarters router then receives the broadcast on its physical interface but cannot
330 Chapter 8: Extending the Network into the WAN
forward that routing update through the same interface to other remote (spoke) routers.
Split horizon is not a problem if a single PVC exists on a physical interface because
this type of connection would be more of a point-to-point connection type.
?– Broadcast replication: With routers that support multipoint connections over a single
interface that terminate many PVCs, the router must replicate broadcast packets,
such as routing update broadcasts, on each PVC to the remote routers.
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