Published: April 29, 2026 October 4, 2021. Facebook, Instagram, and WhatsApp went completely dark for roughly six hours — all at once. The servers were fine. No bad deploy. A single command run during routine maintenance withdrew every one of Facebook's BGP routes. The internet forgot how to reach Facebook's data centers. Traffic had nowhere to go. Facebook ceased to exist on the internet. The servers were running. The load balancers were healthy. Everything was fine. Requests just couldn't get in. That's what happens when traffic distribution breaks at the routing layer. No matter how well-built the system behind the load balancer is — if requests can't reach it, none of it matters. Not all load balancers work the same way. Some look only at the outside of a packet and route it fast. Others open the packet, read what's inside, and decide based on the contents. In Part 1, the trade-off was clear: L4 is fast because it stays ignorant, L7 is precise because it pays to know. Load balancers face the same choice.…