Covered in previous blog posts (Part 1 and Part 2), the concept of the Virtual Modular Switch (VMS) is clearly an advantage for networks of medium to large scale. As we move into huge networks where multiple modular switches are needed, this advantage reduces to the point where it is a matter of personal preference whether to implement using VMS or multiple chassis.
When the odds are even, this preference can come down to a matter of cost of equipment, cost of operating the equipment, certain network KPIs that need to be met or any other parameter that the network facilitator will care about.
The Mellanox implementation of VMS is based on our own ASIC design known as SwitchX. It is used as the fabric element in each of our Ethernet (and InfiniBand) product line of switches. SwitchX carries 36 high speed interfaces of standard 40 GbE which when used in a non-blocking fat tree topology, allows 18 ports to be used for external interfaces and 18 ports to be used as internal interfaces towards the spine layer of the VMS fat tree. Having 36 ports on each of the spine elements allows as many as 36 leaf elements. The total number of external ports in a non-blocking two tier VMS is 36*18=648.