On the 100G Ethernet interface the Loss Of Block Lock (LOBL) alarm is raised when it is not possible to lock onto the sync. header within the 64b/66b line coded blocks.
As the serial stream of blocks are distributed over the 20 PCS lanes during transmission the LOBL alarm should be reported within a specific PCS lane. In cases where a LOBL is reported the receiving device will be unable to correctly receive the 66b blocks from that specific PCS lane and therefore be unable to reconstruct the total Ethernet frame. In this case the total Ethernet link between the devices would fail.