How the internal network is arranged.
WHAT THE TRADITIONAL ARRANGEMENT WAS
Three tiers: access, aggregation and core.
WHAT IT OPTIMISED FOR
Traffic entering and leaving the facility.
WHAT CHANGED
Modern workloads generate far more traffic between machines than to the outside.
WHAT THAT PRODUCED
Flatter designs, optimised for traffic between any two points.
WHAT A LEAF AND SPINE DESIGN IS
Leaf switches connecting servers, each connected to every spine switch.
WHAT THAT PROVIDES
Equal path length between any two servers Predictable latency Capacity added by adding spines
WHAT OVERSUBSCRIPTION MEANS HERE
More server capacity connected to a leaf than the leaf has upward.
WHAT RATIO TO AIM FOR
Depends on the workload, measured rather than assumed.
WHAT EAST-WEST AND NORTH-SOUTH DESCRIBE
Traffic between servers, and traffic in and out.
WHY THE DISTINCTION MATTERS
It determines where capacity is needed.
WHAT TO MEASURE
Actual traffic patterns, before designing.
WHAT TO PLAN FOR
Growth in the number of connections, not only bandwidth.
