Any enclave manifests by its very existence a cause of a certain degree of
'extraterritoriality' or rather dilution of sovereignty to the host state.
By recognising its existence, the host state is obliged to provide/allow
access from the home state to it. This is the same in international law, as
in domestic law. If you own a property and sell off a section of the middle
of that property, you are obliged, by that sale, to allow access between
that sold section and the outer edge of your property/a public road.
The India/Portugal ICJ case over Dadra & Nagar Haveli had an article by a
law professor going trough the legal instances in the domestic laws of
dozens of countries.

Brendan