nVidia SLI / GL_NVX_linked_gpu_multicast extension transfers over SLI bridge or PCI bus?

When using the SLI GL_NVX_linked_gpu_multicast extension I am curious if the transfers between GPU’ use the SLI bridge or PCI bus or may use both?

As I am examining best case usage scenarios with multi-gpu’s simulation/rendering/VR and knowing what might be transferred over what would be useful information that I haven’t seen anywhere?