This read-only counter reports the number of seconds since this IS-IS circuit last transitioned into the 'up' state. It gives an administrator a quick way to see how long an interface's IS-IS adjacency-capable state has been stable, which is useful when correlating recent flaps with other network events. For example, seeing csIsisCircUpTime reset to a small value on a core link that should have been stable for weeks would prompt an operator to check for a recent physical link flap or IS-IS process restart.