A Fraggle attack is a Layer 4 reflection and amplification vector, one of the volumetric classes a thorough DDoS test is built to exercise. It is the UDP cousin of the Smurf attack: the attacker spoofs the victim's IP in UDP packets aimed at the echo (port 7) or chargen (port 19) service on a network's broadcast address. Every host on that segment that runs the service replies to the spoofed source, so a single packet fans out into a flood directed at the victim.
Why it matters in DDoS testing
Fraggle is a classic that survives only where directed broadcasts and legacy diagnostic services are still enabled. Testing confirms that routers drop directed broadcasts (the no ip directed-broadcast default), that echo and chargen are disabled internally, and that ingress filtering blocks the spoofed sources before they leave the edge. The reflection mechanics shared across these vectors are detailed in Understanding DDoS Attack Vectors.