inter-as mpls vpn is a way two providers peer in order to
provide a seemingly continous service to a given customer.
a simpler way is when peering happens between the two provider's pe
routers directly within the bgp vpn afis. the remote
provider's vpn prefixes will be visible on the local core as if it was
originated by the border pe in terms of next hop, and as path will
be there to indicate that the route came from the remote core.
alternatively, there is a more robust way when they peer on those
border pe routers just in the labeled unicast afi,
where they exchange only loopback reachibility with labels,
and also peer in vpn afis between their route reflectors. in this way each customer
route will be visible with the remote originating pe router's loopback
and in case of the inter-as link goes down, just the impacted remote loopbacks will reconverge,
the vpn afis won't be affected at all.
alternatively, one can choose to mix the above two peering schemes when one provider chooses
to do all the trick at it's border pe.
also note that not every kind of vpn could be carried with the simpler way.