It looks as though I may be having a problem with STP on the Cumulus switches.
DistA is root and DistB is backup. With a sustained ping running (from DistA to AccessC), I disabled the link between DistA and AccessC. I lost one ping initially, and the route via DistB kicked in. However, it worked for about a minute, but then timed out.
Is there some sort of timer that needs to be manipulated?
All three switches in the above scenario described are Cumulus switches.
DistA =========== DistB
However, I substituted AccessC with a different 3rd party open OS (Linux-based) running MST, and the failover worked fine with the Cumulus distribution switches.