Everyone loves a battle of heavyweights, so it’s no surprise that the industry has been captivated by the ebb and flow in the network virtualization space between Cisco’s Application-Centric Infrastructure (ACI) and VMware’s NSX or (as we like to say: Cisco ACI vs VMware NSX).

“This is definitely a hot topic,” says Gartner research director Andrew Lerner. “In terms of when clients call into Gartner and want to talk about networking and data center networking, NSX versus ACI is the No. 1 topic. It’s the biggest topic we get among the networking analysts at Gartner.”

Before we take a look at who may or may not be winning this Texas Death Match of SDN, it’s worth noting that both combatants were initially outliers. VMware’s NSX platform is the direct result of its $1.2 billion acquisition of Nicira Networks three years ago. Cisco ’s ACI has its roots in Insieme Networks, which Cisco funded as a spin-in startup, then acquired for $863 million in 2013.

Cisco and VMware had been partners of sorts, but that began to change when the Nicira purchase landed VMware in Cisco's networking arena. For its part, Cisco was initially behind the curve with its SDN portfolio, but the launch of ACI in July 2014 made it a formidable opponent.

Click for sidebar: NSX vs. ACI, Tale of the Tape

While it's entertaining to compare the two (see sidebar), NSX and ACI aren’t an apples-to-apples comparison, because they were designed to perform different functions.

“I do think a false dichotomy has been created,” says VMware’s Martin Casado, senior vice president of VMware’s network and security business unit, and the founding CTO of Nicira. “These are fairly separate products. ACI is a control plane for primarily physical fabric management that’s attached to one set of switches, which are Cisco’s switches.”

At a high level, NSX, which officially became available in October of 2013, is focused on virtual networking on the edge. According to SDxCentral’s 2015 Special Report: Network Virtualization in the Data Center, authored by SDxCentral analyst Scott Raynovich, NSX has three primary use cases: IT automation, microsegmentation, and application continuity.

ACI, on the other hand, is an SDN architecture that relies on a policy-based operating model, according to the 2015 Special Report: Network Virtualization in the Data Center. ACI uses pre-defined application requirements and policy profiles to automate the provisioning of the network, application services, security policies, tenant subnets, and workload placement.

“NSX is an application. It’s not a network,” says Cisco’s Frank D’Agostino, CTO of Cisco’s Insieme Business Unit. “ACI is a network that is focused on applications, and NSX is an application. As a result of that, ACI as a network for applications can run NSX better than any other network in the world.”

According to Cisco’s first quarter earnings report earlier this month, it now has more than 1,100 ACI customers compared to 900 in the previous quarter. In order to use ACI on Cisco’s Nexus 9000 switches, Cisco customers need to deploy the Application Policy Infrastructure Controller (APIC).

During VMware’s most recent earnings in October, it reported 900 paying customers in the third quarter compared to 700 in the previous quarter.

Based on the length of time that both products have been available for purchase, Gartner’s Lerner gave ACI the edge over VMware in paying customers prior to Cisco’s earnings report.

“It was a 2:1 ratio, but it appears to be evening out,” Lerner wrote in a recent email to SDxCentral. “We were seeing more ACI deployments as of several months ago. However, these recent numbers indicate that adoption may be closer in [recent] weeks and months.”

Lerner says Gartner’s ACI and NSX numbers can fluctuate based on the time of the year, such as during Cisco Live or VMworld events.

From SDxCentral's 2015 Special Report: Network Virtualization in the Data Center. Click for larger view.

By contrast, the 2015 Special Report: Network Virtualization in the Data Center cites NSX as the product that is most often being used or evaluated by customers with Cisco in second place. It’s the second year in a row that the SDxCentral NV survey has NSX at the top of its leaderboard. (See image at right).

Lerner, Casado, and D’Agostino say the real measure of success for both NSX and ACI is the number of customers who are actually deploying the technologies, but, of course, they have different takes on those numbers as well.

“Now there’s a huge difference between paying customers and customers who are actually deployed and in production,” Lerner says. “So I think that’s a really, really key point to call out.”

Lerner estimates that 10 percent to 20 percent of Cisco’s ACI customers are in production while “probably” 15 percent to 20 percent of the NSX customers are in production.

"Cisco and VMware combined have over 1 million customers, but only 300 of them are actually doing this in production,” Lerner says. “That kind of puts into context the echo chamber around SDN.”

Cisco’s D’Agostino says the production numbers are “north” of Lerner’s estimates, and that production for ACI is rapidly accelerating across all segments. With ACI only being out for more than a year it has taken time for proof-of-concept trials, performance testing, and vetting of ACI, D’Agostino says.

“I would say the reality is, where are customers voting with their dollars and where are customers voting with revenue?” D’Agostino says. “Based on our results, I think it’s clear what they are doing.”

In line with Lerner’s estimate, VMware said in its third-quarter earnings report that it had 150 customers in production. On average, Casado says that 25 to 50 NSX customers are going into production each quarter.

“NSX sales are going like crazy, so of course the number of sales is going to exceed the number of customers that are in production,” Casado says. “I think this is probably the most important thing I’ll say. The currency of a new technology adoption is production customers.”

Casado cites the public testimonials and use cases by NSX customers at events such as VMworld, as well as the 80 customers that have spent more than $1 million on NSX, as proof of NSX’s success.

“I believe all of that really shows the level of growth that NSX has,” he says.

D’Agostino’s counterpunch is that Cisco’s next-generation data switching portfolio, which includes the Nexus 3000 and 9000 switches and ACI, is now at a $2 billion run rate, with over $500 million in revenue in the first quarter, a 140 percent year-over-year increase.

“This performance is much stronger than that of our competitors, who claim they are outpacing and outperforming us,” he says. “This is the fastest ramping data center product that we’ve ever had.”

However, a debate about Cisco and VMware's numbers wouldn't be complete without a discussion of how those numbers are reached.

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