SERVER-25158, we bumped the configsvr mode number in isMaster when the config server has featureCompatibiliityVersion "3.4". This prevents a 3.2 mongos from starting up against a FCV("3.4") cluster. Existing versions of mongos will get stuck in an infinite loop during sharding initialization as they attempt to connect to the config server replica set:
While looping, the mongos will repeatedly log messages such as the following:
Instead, the mongos process should quickly exit with a non-zero exit code and a useful error message, without ever making its routing service available. The error message should display a "MustUpgrade" error code and should link to the documentation for the 3.2=>3.4 upgrade process.