DRBD on RHEL/CentOS 6 fails to sync with UnknownMandatoryTag error

We use DRBD as part of Linux HA designs. I was recently tasked with setting up DRBD 8.4 for a new customer installation. I chose a complicated, 64-character random string as my shared secret, and configured both nodes.

Big mistake.

DRBD appeared to start, but both nodes were in “Standby” mode, and drbd wouldn’t even start listening on TCP port 7788.

When I ran “drbdadm connect drbd0” on one of the nodes, I got an error like this:

drbd0: Failure: (126) UnknownMandatoryTag

I changed the shared secret to something less complex (“supersecrit”), restarted drbd on both nodes, and suddenly drbd started, the nodes synced, and my day improved :)