An online system monitoring technique quickly and efficiently identifies failures or other system errors arising during operation of an intermediate network node, such as a network switch. The technique comprises Keep Alive Buffer packets/cells ("KABs") that exercise data and control paths extending from every ingress port to every egress port in the switch. By exercising the data and control paths, the KABs enable testing of, and ensuring against, component failures, missing modules or other types of failure that can be detected as soon as possible, to thereby prevent data flow backup or other performance degradation in the switch.