A service provider deploys a VPLS instance across four PE routers using an LDP-signaled full mesh of pseudowires. During testing, an engineer observes that a broadcast frame injected at PE1 is correctly received by CE devices attached to PE2, PE3, and PE4 exactly once, with no duplicate frames or forwarding loops — even though no spanning tree protocol is running in the provider core. Which VPLS mechanism explains this loop-free behavior across the pseudowire mesh?