To support upgrade to config servers as replica sets (CSRS) from sync cluster connection config servers (SCCC), it is necessary for shard servers and mongos nodes to dynamically switch their catalog manager implementation based on the config server protocol reported by the config servers.

To this end, config servers running the CSRS protocol should report a new top level field in their response to ismaster, configsvr: 1 to indicate that shard and mongos nodes should communicate with the config servers using the config server replica set protocol version 1. If the field is missing or has value 0, those nodes should instead communicate using the SCCC protocol.