In document "Add Shards to the Cluster", it says:

After version 2.0.3, you may use the above form to add replica sets to a cluster. The cluster will automatically discover the other members of the replica set and note their names accordingly

Now I can use sh.addShard("sfo30.example.net") instead of sh.addShard("sh0/sfo30.example.net,sfo31.example.net,sfo32.example.net") which is very handy.

However, if replica set 'sh0' contains only one member, say 'sfo30.example.net', when I run sh.addShard("sfo30.example.net"), I will get:

{ "ok" : 0, "errmsg" : "host is part of set: sh0 use replica set url format <setname>/<server1>,<server2>,...." }

which force me to use the old url format sh0\sfo30.example.net.