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Frame Relay Configuration 263

In configuring a subinterface, you must specify whether it is point-to-point or multipoint. Point-
to-point subinterfaces establish a single PVC between two end points. Each interface is its own
subnet and has a single DLCI configured. Broadcasts do not present an issue here because the
routers treat the subinterface as a dedicated circuit between the two points.

Multipoint subinterfaces can be used with multiple PVC configurations on a single
subinterface. All participating PVC connections are members of a common subnet. Multiple
DLCIs would be configured here. However, split horizon dictates that no routing updates can
pass in from and then out of the same subinterface.

Frame Relay Configuration

The basic configuration of Frame Relay is quite simple. In many cases, the configuration can
be as simple as setting the encapsulation and putting an IP address on the interface. This enables
inverse-ARP to dynamically configure the DLCI and discover neighboring routers across the
cloud. However, configurations are rarely that simple. Although basic functionality can be
achieved in this manner, more complex procedures are necessary for hub and spoke
subinterface configurations dealing with point-to-multipoint implementations.

Configuration of Frame Relay can be accomplished in a few steps, as follows:

Step 1

Determine the interface to be configured.

Step 2

Configure Frame Relay encapsulation.

Step 3

Configure protocol specific parameters.

Step 4

Configure Frame Relay characteristics.

Step 1: Determine the Interface to Be Configured

The first step is to decide which interface should be configured for Frame Relay functionality.
This should be a fairly easy decision. The interface that interfaces the Frame Relay network is
obviously the one needing the configuration. Once the interface has been selected, you should
change to the appropriate interface configuration mode in the router.

Now is the time to decide whether subinterfaces should be implemented. For a single point-
to-point implementation, it might not be necessary to use subinterfaces; however, this
implementation does not scale. If future sites are planned, it is best to use subinterfaces from
the beginning.

To create a subinterface, simply enter the command to change to the desired interface, and the
interface is created. For example, to create subinterface 1 on Serial 0, use the command
interface serial 0.1. Once this command is entered, the interface is created and the router
prompt is Router(config-subif)#.