Hi Y Sheng,
Below are 2 links that will help you with your questions:
Designing an HPC Cluster with Mellanox InfiniBand Solutions
Designing an HPC Cluster with Mellanox InfiniBand Solutions
Here you can find several examples of how to design a cluster.
http://www.mellanox.com/clusterconfig/
Here you will fine the Mellanox InfiniBand Configurator which is an online tool to configure clusters based on a FAT Tree Topology with two levels of switch systems.
Marlon