RTBH (remotely triggered black hole) is a network-layer DDoS mitigation in which an operator uses BGP to null-route all traffic destined for a targeted IP address at the network edge. The targeted destination is deliberately dropped so the attack traffic never transits the core network, protecting every other service on it.
Why it matters in DDoS testing
RTBH is a blunt, last-resort control, and its defining property is collateral cost: it denies all traffic to the black-holed address, legitimate users included. So the mitigation itself can be the outage. A DDoS test that triggers an RTBH scenario characterizes two things: the diversion latency (how quickly the route propagates) and the blast radius (whether black-holing a single target also strands services that share the prefix). Carrier-level RTBH and more selective alternatives like BGP Flowspec sit on the same spectrum of trade-offs.
For where edge mitigations fit in a full assessment, see The Complete Guide to DDoS Testing.