Para se conectar a um conjunto de réplicas Primário, use o shell mongo
--host opção:mongo --host replicaSetName/host1[:porthost1],host2[:porthost1],host3[:porthost3],etc
Por exemplo:
$ mongo --host rs1/john.local:27019,john.local:27018
MongoDB shell version: v3.4.9
connecting to: mongodb://john.local:27019,john.local:27018/?replicaSet=rs1
2017-10-12T14:13:03.094+0000 I NETWORK [thread1] Starting new replica set monitor for rs1/john.local:27019,john.local:27018
2017-10-12T14:13:03.096+0000 I NETWORK [thread1] Successfully connected to john.local:27019 (1 connections now open to john.local:27019 with a 5 second timeout)
2017-10-12T14:13:03.096+0000 I NETWORK [thread1] Successfully connected to john.local:27018 (1 connections now open to john.local:27018 with a 5 second timeout)
rs1:PRIMARY> db
test
rs1:PRIMARY>
Observação: Das versões 3.4.2 a 3.4.10 havia um bug (SERVER-28072) que impedia a especificação do db depois ao usar --host ou --port.
