This counts the total number of times this BFD session has transitioned from up to operationally down since it was created, serving as a cumulative flap counter rather than a snapshot of current state. An admin treats a rapidly incrementing value here as evidence of an unstable link or an aggressive timer configuration triggering false failure detections, distinct from a session that is simply down right now but has never flapped before. For example, cienaCesBfdSessionDownTimeCount reading 47 on a session that has only existed for a day would point the admin toward a marginal physical link or an overly aggressive detection interval rather than a single clean failure event.