> While acknowledging that a data center may make any rules it likes, I > am asking nanog how common this practice is.
"data center" is too amorphous a term to be used here. private data centers owned by banks or insurance companies aren't relevant at all. telco motels aren't really data centers but the issue does come up there. someone like exodus or qwest or at&t or uunet or abovenet would be very likely to prevent their customers from directly cross-connecting. mae-west (55 s market) won't allow it either. paix, equinix, switch and data, and other "neutral colos" won't allow it to occur without a fee but the fees are reasonable (unlike, say, the cross connect fees at mae-west.) there's no answer to the question, as posed. "can you be more specific?"
