ACI, however, can tie both physical and virtual environments together and treat them equally from a connectivity, security, user experience, and auditing perspective. ACI's blend of hardware and software components allows you to address the data center network as a whole with a complete solution, rather than stack a house of cards using disparate components, each handling some small portion of total requirements.
The major consideration when deploying ACI is the insertion strategy into existing networks. Greenfield deployments are rare, and the average network has existing equipment still very capable of doing its job, and receiving support. Because ACI utilizes a combination of hardware and software there is a misconception that it requires a "forklift upgrade" of existing network hardware.
In reality, ACI has a very low entry point from a hardware, software, and cost perspective. This entry typically includes two low-cost, high-performance spine switches (32 ports of line-rate 40G each), which provide connectivity between leaf switches for interconnectivity, as well as other ACI functions. It also includes two leaf switches (48 ports of line-rate 1/10G with 12 port of 40G uplink) for connectivity of servers, L4-7 services appliances, external network, WAN, etc. Lastly, the Application Policy Infrastructure Controller (APIC) cluster, and all required licensing would complete the entry point.
This is all packaged as a starter kit bundle, which is priced at about the same as the server purchase requirement would be for the control system of a software-only solution such as NSX. For example, the current pricing on an ACI starter kit is below $125,000 average street price (ASP). Compare this to the various x86 hosts required to manage, monitor, and provide gateway functions for NSX, along with the required host and feature licensing.
Because ACI is compatible with all modern data center network standards such as 802.1Q, BGP, OSPF, etc. this entry level ACI option can be integrated with switching systems from any vendor based on standard protocols. Even when integrated with existing environments, ACI provides the same automation, application visibility, security segmentation, and service automation capabilities it would in a greenfield deployment.
Since ACI and NSX aren't really in competition -- one an SDN solution, one an NFV solution -- might I want both? The answer is yes; in some cases implementing both may be useful. ACI is arguably the most robust and automated network transport solution, while NSX offers hypervisor-based load-balancing and firewalls, especially when paired with third-party VMware partners for more than basic functionality. Because of that, VMware NSX's suite of NFV tools can be complementary to Cisco ACI's comprehensive SDN architecture.
NSX at its heart is simply another virtualized application running within the hypervisor. ACI is a network focused on the individual requirements of an application. When using the two together, VMware NSX would simply be segregated as another application within the ACI policy model. ACI would then cover the scope of automation, security, auditing, and micro-segmentation that NSX does not handle -- the bare-metal servers, physical appliances, Linux containers, the VMware kernel and vMotion ports, etc.
In this model, NSX is providing micro-segmentation for VM-to-VM traffic, while ACI utilizes NSX as a piece of a holistic security stance that encompasses today's real-world workload mix. Additionally, ACI alleviates the need for gateway servers by providing gateway, bridging, and routing functions for untagged, VLAN, VxLAN, and NVGRE on any port. This means that the 50%+ customers using multiple hypervisors can easily connect heterogeneous virtualization solutions seamlessly without additional hardware.
As with any major technology shift and product decision, it's important to gain a strong understanding of what the products offer, and what the business goals are for technological shift. It's best not to look at ACI and NSX as competing solutions, because they truly aren't. If your business requires a dynamically provisioned, scalable and programmable network, ACI is the leading choice. If your business requires hypervisor-level micro-segmentation for VM-to-VM traffic, NSX is a solid choice. If both are required by the business, the two can work together to meet those requirements.