Despite a robust network virtualization platform and existing customers dating back to the Nicira days, NSX has its critics. A chief concern expressed by the engineering community surrounds NSX's lack of communication with network switching hardware, relying heavily on vSwitch programmability to fulfill network virtualization goals.
While VMware has done its best to contradict this notion, the fact remains that NSX simply does not have specific insight into all of the network hardware forming the underlay fabric the NSX overlay rides on, which has implications for everything from traffic engineering to fault isolation and load distribution. That's not to say NSX has no knowledge of the physical network, but rather that most of what NSX does know is inferred.
VMware's official blog site goes into depth explaining that NSX can help isolate problem domains to point administrators in the right direction when troubleshooting an application problem, including a problem with the physical network. But to NSX, the physical underlay network is largely a cloud where tunnel packets enter on one side and exit on another.
In addition to the network hardware criticism, early reports from organizations exploring NSX cite pricing as an adoption barrier. VMware and 80% owner EMC are no stranger to complex SKU build sheets and costly licensing schemes that make IT organizations wince, and reportedly NSX is no exception. That said, folks within VMware say they are aware of customer concerns in this area and wish to avoid another "vTax" public relations debacle. Suffice it to say, potential NSX customers need to stay tuned in this area.
The name Cisco chose for its SDN effort, Application Centric Infrastructure (ACI), is significant because it sends a message. With ACI, Cisco is focused on shaping network infrastructure to the needs of specific network applications.
Does that include network virtualization? Certainly. But with ACI, network virtualization isn't the whole story. Rather ACI is an entire SDN solution wrapped around the idea that IT applications are the most important thing in an organization.
In that sense, it's difficult to compare NSX and ACI directly. While there is some functional overlap between NSX and ACI, ACI doesn't merely answer the question, "How can a network be virtualized?" Rather, ACI answers the question, "How can networking be transformed to revolve around an application's needs?"
As complex and nuanced of a solution as NSX is, ACI is both broader in scope and more novel in approach. An organization could conceivably run NSX over ACI but not the other way around.
All of that said, ACI as an entire solution isn't shipping yet. The ideas are all there. Significant amounts of code have been written. Product components have been named. But for customers, ACI doesn't really exist. Customers who invest in available ACI components are investing in roadmaps that promise a complete ACI solution delivered over the 2014 calendar year.
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