EDD + WAN handoff
The edd is the line on the floor where your cabling stops and the carrier's begins. It is the one box every cable headed for the Internet has to pass through on its way out. Think of it as the front door of the building: nothing reaches the outside world without going through it.
Because everything funnels through the edd, it is also the place you can least afford to lose. If the edd goes offline, it does not matter how healthy your racks are inside — every customer loses the Internet at once, no exceptions. That is why you watch its spare capacity: run too many uplinks into too small a edd and you have quietly built your whole facility on one fragile doorway.
Detailed explanation
The edd is the single ingress/egress boundary between your facility and the carrier. WAN ports on it accept uplinks — consumer fibre, business fibre, datacenter peering — and every north-south flow crosses it. It is the handoff where your AS meets theirs; the smartjack/MPOE in physical terms.
Edd offline = facility-wide north-south outage. No internal redundancy buys you out of it, so headroom is the lever: EddHeadroom tracks WAN-port and capacity slack against your committed uplinks. Saturate it and new peering has nowhere to land; lose the box and the whole site goes dark to the outside. The reachability chain is only as strong as this link.