The excess convergence ratio is a core indicator to measure whether the data center network bandwidth is sufficient, and directly determines how much throughput the application can actually obtain during peak traffic times. Many people memorize the 1:1 conclusion, but they don't know how to calculate their own scenario. This article explains the caliber, algorithm and value in one go.

First distinguish between north-south and east-west directions

Convergence occurs in the upstream direction: the Leaf downstream port is connected to the server, and the upstream port is connected to Spine. The total downstream bandwidth divided by the total upstream bandwidth is the convergence ratio. North-south-based services are sensitive to egress bandwidth, while east-west traffic such as virtual machine migration and storage replication are more sensitive to Leaf uplink. Before evaluation, you should first clarify the traffic composition to avoid using the wrong caliber to calculate figures that deviate from the actual value.

Calculation methods and commonly used values

For example: A Leaf has 48 25G downlink ports and 8 100G uplink ports. The total downlink port capacity is 1200G and the total uplink port capacity is 800G. The convergence ratio is 1.5:1. It is generally recommended that distributed storage and high-performance computing be configured at 1:1 to ensure no blocking at any time; general virtualization can accept 2:1 to 3:1; the parameter network for AI training should be strictly 1:1, and burst synchronization traffic is extremely sensitive to queuing.

How to save uplink costs

  • The uplink ports are evenly allocated to each Spine to prevent a single Spine from being overloaded and forming a hidden bottleneck.
  • Short-distance priority in the same park40G DAC high speed cable or 100G DAC high speed cable, save money on modules
  • Reserve 1-2 uplink ports as margin for upgrade to avoid cable replacement when the bandwidth is increased.

The reasonable value of the convergence ratio should be combined with historical monitoring and pressure measurement iterative calibration. EB-LINK can output a list of modules and cables according to the target convergence ratio. Consult 0755-83179002.