1. add software defined network, where transport and signaling is done by vendor-specific underlay, possibly across multiple redundant uplinks
2. term "external" is really vague as modern networks have blended boundaries. Things like availability zone, region make dc-dc connection irrelevant, because at any point of time you will be required to failover to another AZ/DC/region.
3. when I think of inter-Datacenter, I can only think of Ethernet. That's really it. Even in Ethernet, what you think of a peer and existing in your same subnet, could be a different DC, again due to software-defined network.
2. term "external" is really vague as modern networks have blended boundaries. Things like availability zone, region make dc-dc connection irrelevant, because at any point of time you will be required to failover to another AZ/DC/region.
3. when I think of inter-Datacenter, I can only think of Ethernet. That's really it. Even in Ethernet, what you think of a peer and existing in your same subnet, could be a different DC, again due to software-defined network.