ACI hardware

In Figure 4, we can see a typical deployment with two spines and three leaves. The Nexus 9500 modular switches are deployed at the top of the topology and act as spines, which in a traditional three-tiered network design would be the aggregation or core switches. Line cards are used to provide the ASICs (application-specific integrated circuitry) required for ACI. There are different line cards available, so make sure you are not purchasing a card that is NX-OS mode only.

The next component is the leaf switches. These are the Nexus 9300-series switches.

The spines connect to the leaves through 40 GE ports, but the spines and leaves are never connected (spine to spine or leaf to leaf).

We can also extend the network, offering greater port density through Nexus 2000-series fabric extenders:

We can also add storage directly to the leaves themselves:

Alternatively, we could use another pair of switches, such as Cisco Nexus 5000-series switches, which would connect to the leaf switches:

The APIC controllers connect to the leaf switches.

The APIC controllers are completely separate from the day-to-day running of ACI. We need them to push new policies. They do not play a part in the functioning of the ACI fabric's data plane, however--only in the control plane.

If we lose one controller, then our tenants’ traffic still flows. If we lose all our controllers, then our tenants’ traffic still flows; we are just unable to push new policies until the controllers are reinstated into the network.

ACI best practice states that we should have a minimum of three controllers. Having three controllers offers high availability; it offers physical redundancy as well as database redundancy. Why not just two controllers, though? Three controllers (well, just an odd number greater than one) work better in a split-brain scenario (one controller disagreeing with another). In such an event, the majority would rule. The controllers use LLDP to find each other, which is part of the process discussed in the ACI fabric overlay section later on in this chapter. We will look at how to use multiple controllers in the troubleshooting section, as the majority of this book uses a much simpler design with just one controller, one spine, and two leaf switches, as seen when we look at the fabric menu later on in this chapter.