I help run an HPC environment that is spread across two data centers, one on-campus and one off-campus. The campus network is already spread across those two data centers, but the HPC environment also includes a couple of VLANs that are completely private.
So, our connection to the campus network is two 100 GbE campus-network ports in one data center, and I have two dark-fiber circuits (two fiber strands per circuit) from one data center to the other, which carry the private VLANs. One circuit is direct; the other goes via a third building.
The flexibility of the dark-fiber service means I can use the same 100GBASE-LR4 transceivers that I use for the connection to the campus network. I don't have to deal with weird transceivers (like single-strand single-lambda Bi-Directional 100 GbE), or custom wavelengths. UIT Facilities gives me a fusion-spliced and tested pair with an LC jack, and it Just Works™.