This read-write TruthValue controls frame blocking behavior while a loopback is active on an Ethernet Network Port: when true, frames in the non-loopback direction matching the test PORT/VLAN are blocked, and loopback-direction frames matching the PORT/VLAN but not the MAC criteria are discarded; when false, no such blocking occurs. Admins enable blocking to prevent live customer traffic from leaking through or colliding with test traffic during an active loopback, keeping the test cleanly isolated. For example, running a BER test on a live customer VLAN, an admin would enable cmEthernetNetPortLoopbackBlockingEnabled so the customer's own non-test traffic on that VLAN is blocked rather than getting mixed in with the loop test.