A butterfly fabric is like a Clos fabric but has five stages instead of three. Figure 13-7 illustrates a small butterfly fabric.
Note
Some network engineers called a butterfly a five- stage Clos. For clarity, this book always calls a three-stage spine-and-leaf a Clos and a five-stage the design described in this section a butterfly.
Figure 13-7 Butterfly Fabric
The left side of Figure 13-7 illustrates a butterfly fabric laid out “flat” so you can see all the connections. There are five hops, or stages, between any two ports in different pods, so this is a five-stage fabric.
The right side of Figure 13-7 illustrates the two parts of a butterfly fabric: the plane and the pod. The spine routers are in a pod and a fabric, which sometimes makes it hard to see the two parts on the drawing of a large-scale butterfly. These two pieces can be treated as separate modules for network design and lifecycle management:
• Once a packet enters a specific plane, it will travel along the plane to the destination. Designers can separate different kinds of traffic onto different planes to simplify quality of service (QoS) and other traffic management problems.
• There is only one path from any plane router to any destination on the fabric. Designers can use this property to steer packets through the fabric.
• A single pod can be removed, repaired, or rebuilt, and re-added back to the fabric—without impacting the rest of the network. Operators can use this to build, test, and replace entire pods while the fabric remains in production.
• An entire plane can be removed and replaced with minimal impact on the network’s operation.
Figure 13-8 illustrates another way butterfly fabrics are sometimes drawn.
Figure 13-8 A Three-Dimensional Illustration of a Butterfly Fabric
Figure 13-8 is harder to decipher but more compact.
Butterfly fabrics can support hundreds of thousands of servers.
Chapter Review
Data center fabrics are a critical part of the Internet’s infrastructure. Most of the shopping, playing, watching, searching, and anything else you do on an Internet-connected app happens on a data center fabric. Most of the traffic passing through the Internet also passes through data center fabrics, as do almost all telephone calls.
Designers almost universally choose three-stage spine-and-leaf fabrics for small- to large-scale fabrics and five-stage butterfly spine-and-leaf fabrics for large- to really large–scale fabrics.
Spine-and-leaf fabrics have unique qualities: they are regular, nonplanar, and use large numbers of parallel links (high fan-out) to support vast amounts of traffic.
One key to doing well on the exams is to perform repetitive-spaced review sessions. Review this chapter’s material using either the tools in the book or interactive tools for the same material found on the book’s companion website. Refer to the online Appendix D, “Study Planner,” element for more details.
Table 13-2 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.
Table 13-2 Chapter Review Tracking
Review All the Key Topics
Table 13-3 lists the key topics for this chapter.
Table 13-3 Key Topics for Chapter 13
Key Terms You Should Know
Key terms in this chapter include web service web server front-end server back-end server DCI IXP route server fabric nonplanar fan-out spine-and-leaf fabric Clos fabric nonblocking noncontending butterfly fabric Concepts and Actions Review the concepts considered in this chapter using Table 13- 4. You can cover the right side of this table and describe each concept or action in your own words to verify your understanding.
Table 13-4 Concepts and Actions
