Shards store a config server connection string in their shard identities. When the config server hosts are modified, this connection string is updated via the RSM when a shard realizes that there is a new configuration for the replica set.

If a shard node is unavailable (but alive) the during the whole reconfiguration of the CSRS, it's configsvrConnectionString will be updated once it reconnects to the cluster and starts exchanging hello requests again.

However, if the node was down and tries to restart after the reconfiguration or restarts before it learns about the new connection string, it will be unable to start up. This is because during sharding initialization, shards must wait to talk to the config server in order to find out the defaultWRC settings and the shard is not open for connections while it does this so the only information the shard has on how to contact the config server is the stale connection string.