3.2.3. Scaling the Clos Topology
3.2.3. Scaling the Clos Topology
A Clos topology can be scaled either by increasing network element port density or by adding more stages, e.g., moving to a 5-stage Clos, as illustrated in Figure 3 below:
Tier 1
+-----+
Cluster | |
+----------------------------+ +--| |--+
| | | +-----+ |
| Tier 2 | | | Tier 2
| +-----+ | | +-----+ | +-----+
| +-------------| DEV |------+--| |--+--| |-------------+
| | +-----| C |------+ | | +--| |-----+ |
| | | +-----+ | +-----+ +-----+ | |
| | | | | |
| | | +-----+ | +-----+ +-----+ | |
| | +-----------| DEV |------+ | | +--| |-----------+ |
| | | | +---| D |------+--| |--+--| |---+ | | |
| | | | | +-----+ | | +-----+ | +-----+ | | | |
| | | | | | | | | | | |
| +-----+ +-----+ | | +-----+ | +-----+ +-----+
| | DEV | | DEV | | +--| |--+ | | | |
| | A | | B | Tier 3 | | | Tier 3 | | | |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| | | | | | | | | |
| O O O O | O O O O
| Servers | Servers
+----------------------------+
Figure 3: 5-Stage Clos Topology
The small example of topology in Figure 3 is built from devices with a port count of 4. In this document, one set of directly connected Tier 2 and Tier 3 devices along with their attached servers will be referred to as a "cluster". For example, DEV A, B, C, D, and the servers that connect to DEV A and B, on Figure 3 form a cluster. The concept of a cluster may also be a useful concept as a single deployment or maintenance unit that can be operated on at a different frequency than the entire topology.
In practice, Tier 3 of the network, which is typically Top-of-Rack switches (ToRs), is where oversubscription is introduced to allow for packaging of more servers in the data center while meeting the bandwidth requirements for different types of applications. The main reason to limit oversubscription at a single layer of the network is to simplify application development that would otherwise need to account for multiple bandwidth pools: within rack (Tier 3), between racks (Tier 2), and between clusters (Tier 1). Since oversubscription does not have a direct relationship to the routing design, it is not discussed further in this document.