There are a couple places in ShardingTest that may cause port conflicts

1. Now that config servers are replica sets, they are started with a "--rest" which uses up a port 1000 + mongod port. Since config servers are started on 29000 by default and shards are 30000 by default. This could potentially cause a shard to fail to start (usually the shards start first so the REST interface fails instead).

2. similarly shards with more than 10 replset members won't start, since their ports are assigned as follows: startPort : 31100 + ( i * 100 )