Run this bash script to start up a shard cluster with one config server. The last command adds a config server as a shard and succeeds. #!/bin/bash DATA_DIR=/data/db TEST_PREFIX= 'addShard-' MONGO_BIN_DIR='' pushd $DATA_DIR echo Deleting old directories rm -rf "$TEST_PREFIX" shard* "$TEST_PREFIX" config* "$TEST_PREFIX" mongos* "$TEST_PREFIX" config_backup* "$TEST_PREFIX" dump* echo Creating directories mkdir "$TEST_PREFIX" shard0; touch "$TEST_PREFIX" shard0/log mkdir "$TEST_PREFIX" shard1; touch "$TEST_PREFIX" shard1/log mkdir "$TEST_PREFIX" config1; touch "$TEST_PREFIX" config1/log mkdir "$TEST_PREFIX" config2; touch "$TEST_PREFIX" config2/log mkdir "$TEST_PREFIX" mongos; touch "$TEST_PREFIX" mongos/mongos.log mkdir "$TEST_PREFIX" config_backup; touch "$TEST_PREFIX" config_backup/log echo Starting shards and config servers "$MONGO_BIN_DIR" mongod -- rest --dbpath "$TEST_PREFIX" shard0 --logpath $DATA_DIR/ "$TEST_PREFIX" shard0/log --port 4000 --fork --shardsvr "$MONGO_BIN_DIR" mongod -- rest --dbpath "$TEST_PREFIX" shard1 --logpath $DATA_DIR/ "$TEST_PREFIX" shard1/log --port 4001 --fork --shardsvr "$MONGO_BIN_DIR" mongod -- rest --dbpath "$TEST_PREFIX" config1 --logpath $DATA_DIR/ "$TEST_PREFIX" config1/log --port 4003 --fork --configsvr "$MONGO_BIN_DIR" mongod -- rest --dbpath "$TEST_PREFIX" config2 --logpath $DATA_DIR/ "$TEST_PREFIX" config2/log --port 4004 --fork --configsvr echo Waiting for config server to start up sleep 3 # let config server start up echo Starting mongos "$MONGO_BIN_DIR" mongos --port 4006 --configdb localhost:4003 --fork --logpath $DATA_DIR/ "$TEST_PREFIX" mongos.log --chunkSize 1 echo Waiting for mongos to start up sleep 3 echo Initiating shard "$MONGO_BIN_DIR" mongo --port 4006 admin <<EOF print( "\nAdding shards" ); db.runCommand( { addshard : 'localhost:4000' , name: 'shard0' }); db.runCommand( { addshard : 'localhost:4001' , name: 'shard1' }); print( "\nAdding config server as shard. This should fail. " ); db.runCommand( { addShard : 'localhost:4004' , name: 'shard2' }); EOF Or you could run this jstest: /** * Test to make sure you can't add a config server as shard */ var st = new ShardingTest({ shards: [{ useHostName: true }], other: { shardOptions: { smallfiles: "", noprealloc: "" } } }); /* Create a new config server */ var configConf = { useHostname: "localhost" , noJournalPrealloc: true , port: 29004, pathOpts: { testName: "test" , config: 4 }, dbpath: "$testName-config$config" , configsvr: "" }; var newConfig = MongoRunner.runMongod(configConf); /* Print some things */ print( "Printing initial config server command line options:" ); printjson(st.config0.getDB( "admin" ).runCommand({ "getCmdLineOpts" : 1})); //res = st.s.adminCommand({addShard: st.config0.host }); print( "Printing newConfig server command line options:" ); printjson(newConfig.getDB( "admin" ).runCommand({ "getCmdLineOpts" : 1})); print( "Printing current config servers" ); printjson(st._configServers); print( "newConfig host:" ); print(newConfig.host); st.printShardingStatus(); /* Test adding a config server as shard */ res = st.s.adminCommand({addShard: newConfig.host }); assert .eq(res.ok, 0); print( "Printing config servers" ); printjson(st._configServers); st.printShardingStatus() st.stop();