Cisco Application Centric Infrastructure (ACI)

Application Centric Infrastructure (ACI).

Interestingly, many SDN offerings began with research that discarded many of the old

networking paradigms in an attempt to create something new and better. For instance,

OpenFlow came to be from the Stanford University Clean Slate research project that had

researchers reimagining (among other things) device architectures. Cisco took a similar

research path, but Cisco’s work happened to arise from different groups, each focused on

different parts of the network: data center, campus, and WAN. That research resulted in

Cisco’s current SDN offerings of ACI in the data center: Software-Defined Access (Cisco

SD-Access) in the enterprise campus and Software-Defined WAN (Cisco SD-WAN) in the

enterprise WAN.

When reimagining networking for the data center, the designers of ACI focused on the

applications that run in a data center and what they need. As a result, they built networking

concepts around application architectures. Cisco made the network infrastructure become

application centric, hence the name of the Cisco data center SDN solution: Application

Centric Infrastructure (ACI).

For example, Cisco looked at the data center world beyond networking and saw lots of

automation and control. As discussed in Chapter 20, “Cloud Architecture,” virtualization

software routinely starts, moves, and stops VMs. Additionally, cloud software enables self service

for customers so they can enable and disable highly elastic services as implemented

with VMs and containers in a data center. From a networking perspective, some of those

VMs need to communicate, but some do not. And those VMs can move based on the needs

of the virtualization and cloud systems.

ACI set about to create data center networking with the flexibility and automation built

into the operational model. Old data center networking models with a lot of per-physical interface

configuration on switches and routers were just poor models for the rapid pace

of change and automated nature of modern data centers. This section looks at some of the

detail of ACI to give you a sense of how ACI creates a powerful and flexible network to

support a modern data center in which VMs and containers are created, run, move, and are

stopped dynamically as a matter of routine.

ACI Physical Design: Spine and Leaf

The Cisco ACI uses a specific physical switch topology called spine and leaf. While the

other parts of a network might need to allow for many different physical topologies, the data

center could be made standard and consistent. But what particular standard and consistent

topology? Cisco decided on the spine and leaf design, also called a Clos network after one

of its creators.

With ACI, the physical network has a number of spine switches and a number of leaf

switches, as shown in Figure 21-9. The figure shows the links between switches, which can

be single links or multiple parallel links. Of note in this design (assuming a single-site design):

 Each leaf switch must connect to every spine switch.

 Each spine switch must connect to every leaf switch.

 Leaf switches cannot connect to each other.

 Spine switches cannot connect to each other.

 Endpoints connect only to the leaf switches.