For each physical interface, a router SHOULD have a configuration option that specifies whether forwarding is enabled on that interface. When forwarding on an interface is disabled, the router:
This feature allows the network manager to essentially turn off an interface but leaves it accessible for network management.
Ideally, this control would apply to logical rather than physical interfaces. It cannot, because there is no known way for a router to determine which logical interface a packet arrived absent a one-to-one correspondence between logical and physical interfaces.