Role of sub-interfaces

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What is the role of a sub-interface?

Say we are configuring OSPF on a router.

Then we have fa 0/0.1 or fa 0/0.2.

What are those for? Why can't we just use regular interfaces?

Thanks.

 

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There are many use cases, one example would be to use one physical interface on a router, to provide inter-VLAN routing (google router on a stick). Each sub-interface can be a member of separate VLAN's, with the interface physically connected to a trunk port on a switch.