To enable the forwarding of broadcast routing updates in a hub-and-spoke Frame Relay topology, configure the hub router with logically assigned interfaces. These interfaces are called subinterfaces. Subinterfaces are logical subdivisions of a physical interface.

In split-horizon routing environments, routing updates received on one subinterface can be sent out another subinterface. In a subinterface configuration, each virtual circuit can be configured as a point-to-point connection. This allows each subinterface to act similarly to a leased line. Using a Frame Relay point-to-point subinterface, each pair of the point-to-point routers is on its own subnet.

Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode:

• Point-to-point – A single point-to-point subinterface is used to establish one PVC connection to another physical interface or subinterface on a remote router. In this case, each pair of the point-to-point routers is on its own subnet and each point-to-point subinterface would have a single DLCI. In a point-to-point environment, each subinterface is acting like a point-to-point interface. Therefore, routing update traffic is not subject to the split-horizon rule.
• Multipoint – A single multipoint subinterface is used to establish multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All the participating interfaces would be in the same subnet. The subinterface acts like an NBMA Frame Relay interface so routing update traffic is subject to the split-horizon rule.

The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network layer address and DLCIs, are assigned to the subinterface.

Multipoint configurations can be used to conserve addresses that can be especially helpful if Variable Length Subnet Masking (VLSM) is not being used. However, multipoint configurations may not work properly given the broadcast traffic and split-horizon considerations. The point-to-point subinterface option was created to avoid these issues.

Web Links Configuring Frame Relay Subinterfaces http://www.cisco.com/en/US/tech/ tk713/tk237/technologies_configuration_ example09186a008009421e.shtml