Currently the systemd unil file mongod.service waits for the network.target to be reached, via the line After=network.target. However, most Linux distributions with systemd support network-online.target, which is a better indicator that the network is up and available for MongoDB. When only waiting on network.target, it is possible that the mongod service fails to start upon system reboot.
See this systemd documentation page for a more detailed explanation.
We suggest changing the mongod.service file to specify: