Starting in 3.4, the use of the deprecated mirrored :binary:`~bin.mongod` instances as config servers (SCCC) is no longer supported. Before you can upgrade your sharded clusters to 3.4, you must convert your config servers from SCCC to CSRS.
To convert your config servers from SCCC to CSRS, see the MongoDB 3.4 manual :v3.4:`Upgrade Config Servers to Replica Set </tutorial/upgrade-config-servers-to-replica-set>`.