3.2.1. Overview

3.2.1. Overview

A common choice for a horizontally scalable topology is a folded Clos topology, sometimes called "fat-tree" (for example, [INTERCON] and [ALFARES2008]). This topology features an odd number of stages (sometimes known as "dimensions") and is commonly made of uniform elements, e.g., network switches with the same port count. Therefore, the choice of folded Clos topology satisfies REQ1 and facilitates REQ2. See Figure 2 below for an example of a folded 3-stage Clos topology (3 stages counting Tier 2 stage twice, when tracing a packet flow):

   +-------+
   |       |----------------------------+
   |       |------------------+         |
   |       |--------+         |         |
   +-------+        |         |         |
   +-------+        |         |         |
   |       |--------+---------+-------+ |
   |       |--------+-------+ |       | |
   |       |------+ |       | |       | |
   +-------+      | |       | |       | |
   +-------+      | |       | |       | |
   |       |------+-+-------+-+-----+ | |
   |       |------+-+-----+ | |     | | |
   |       |----+ | |     | | |     | | |
   +-------+    | | |     | | |   ---------> M links
    Tier 1      | | |     | | |     | | |
              +-------+ +-------+ +-------+
              |       | |       | |       |
              |       | |       | |       | Tier 2
              |       | |       | |       |
              +-------+ +-------+ +-------+
                | | |     | | |     | | |
                | | |     | | |   ---------> N Links
                | | |     | | |     | | |
                O O O     O O O     O O O   Servers

                  Figure 2: 3-Stage Folded Clos Topology

This topology is often also referred to as a "Leaf and Spine" network, where "Spine" is the name given to the middle stage of the Clos topology (Tier 1) and "Leaf" is the name of input/output stage (Tier 2). For uniformity, this document will refer to these layers using the "Tier n" notation.