It happens when it's mutually beneficial, regardless of whether the traffic is balanced. I have (free) peering w/ a few networks that are wildly off-balanced (o

It happens when it's mutually beneficial, regardless of whether the traffic is balanced. I have (free) peering w/ a few networks that are wildly off-balanced (on the order of 5:1 at times) simply because "free" is still cheaper than what I'd pay to send it to them over another (paid) transit link.