This TruthValue controls frame-blocking behavior while loopback is active on the port: when true, it blocks non-loopback-direction frames matching the loopback PORT or VLAN, and discards loopback-direction frames that match the PORT or VLAN but not the MAC address criteria; when false, no blocking occurs. An administrator enables this to prevent live customer traffic from leaking through or colliding with the loopback test rather than letting both traffic types mix on the wire. For example, turning this on during a service-affecting loopback test ensures normal customer frames matching the test's VLAN do not get forwarded alongside the looped test frames.