Trying to Build a 40 Gbps Switch From Several, Smaller Switches.

Can't You Build a 40 Gbps Switch Out of Multiple Lower Speed Switches?

You can try to build the features of the ForeRunner® ASX-4000 using smaller switches, but it is more expensive in terms of capital and network engineering costs. Further, building larger backbones with smaller switches usually results in dramatically lower performance. Vendors of smaller switches try to provide either the physical port count or the port count and the non-blocking capacity of the ASX-4000. The number of smaller switches required depends on the goal. 

For example, providing 64 OC-12c/STM-4c ports with 20 Gbps switches connected together, as shown below, will result in three switches interconnected with blocking performance beyond 5 Gbps. 

Three 20 Gbps Switches Providing 64 User Ports of OC-12c 

Using Three 20 Gbps Switches To Provide 64 User Ports of OC-12c. 

While you may realize the port count, this configuration incurs additional rack space, VC space, provisioning and capital costs, as well as a blocking solution. 

To provide the port count and non-blocking capacity of the ASX-4000 by using several 20 Gbps switches, requires six 20 Gbps switches, as the following illustration shows: 
 

Using Six 20 Gbps Switches To Provide 40 Gbps Non-Blocking Capacity 

Using Six 20 Gbps Switches To Provide 40 Gbps Non-Blocking Capacity. 

Beyond the capital costs of six switches, building a 40 Gbps switch out of multiple lower speed switches results in network engineering costs, such as: 

The following table compares the number of switches and cost figures (given the same base-price per OC-12c/STM-4c port) for a non-blocking 40 Gbps backbone, by switch size: 
Switch Capacity: Number of Switches: Relative Price: Approximate Cost:*
40 Gbps One (1) the same US$512,000
20 Gbps Six (6) 300% increase US$1,536,000
  *With switches offering OC-12c/STM-4c at US$8,000 per port. 

By providing 40 Gbps of non-blocking capacity in a single unit, the ForeRunner ASX-4000 allows customers to avoid the capital and network engineering costs required by deploying multiple lower capacity switches.