"How are the Data Centers connected to each other?

The two Data Center Parks are both connected to Frankfurt (FFM) and each other with dark fiber. Thus, a redundant loop is formed, which ensures the availability of a Data Center should one of the connections fail. The n10 Gbit/s connections provide ample bandwidth between the Data Centers.

The bandwidth of the connections between Nuremberg-Frankfurt, Nuremberg-Falkenstein and Falkenstein-Frankfurt are at least 120 Gbit/s. Through the Frankfurt location data is transported to the peering partners at DE-CIX and also to the uplinks Noris, GLBX, Aixit, AMS-IX, Init7 and Level3. At the Nuremberg location there are connections to Noris, KPN, Init7, Level3 and N-IX.

In each Data Center several Juniper EX Core switches, each with 64x 10 Gbit/s ports, are operated and bundle the streams of the Data Center to the n10 Gbit/s backbone and then over the various uplinks. "

And here is a list of their peerings: https://www.hetzner.com/unternehmen/rechenzentrum/