These sub interfaces can be used in frame relay environment.You can create a point to point or point to multipoint sub interface to connect to your remote sites using FR.
To add a little to the explanation from Nambi: certain encapsulations like Frame Relay do support configuration of subinterfaces. But other serial encapsulations like PPP or HDLC do not support configuration of subinterfaces.
The main use of subinterfaces in Frame Relay is to be able to separate out various PVCs that are assigned over the serial link. If you have a Frame Relay connection over a serial interface you might have 4 PVCs from the provider on that link. Without subinterfaces all PVCs would operate on the physical interface in a multipoint mode. If you configure subinterfaces then you could create 4 separate point to point subinterfaces and assign a PVC to each subinterface which would operate in point to point mode rather than multipoint mode. There are sometimes advantages to operating in point to point mode such as the operation of split horizon.
Adding to Rick's comment that you can run PPP over Frame-relay for the purpose of Authentication since Frame-relay by default doesnt support features like (Authentication,Link Quality,Reliable transmission).
Running PPP over FR could enable these features.
Regards,
Mohamed Sobair
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