Cisco has consolidated its SDN strategy around the Application-Centric Infrastructure and the Application Policy Infrastructure Controller (APIC), but that decision left a few other products and acronyms dangling. One such is nLight, the converged control plane for packet and optical networking.

It turns out nLight isn't dead, but it's now considered a transitional phase on the way to optical SDN, reports Heavy Reading analyst Sterlin Perrin. He got the lowdown at Cisco's North American Packet Optical Networking Conference (PONC), held in mid-May in San Jose, Calif.

Optical networking might be an obscure corner of Cisco to some, but it got a boost in recent years under the High-End Routing and Optical (HERO) group, as Cisco got aggressive with combining router and optical-transport technologies.

Along those lines, Cisco introduced nLight in late 2012 as a merging of the IP and optical control planes. That's a common theme among optical networking vendors such as Ciena and Cyan, and it's an idea that can fit in a software-defined networking (SDN) scheme. NLight was specifically applied to the NCS line of packet-optical transport systems that Cisco introduced in September.

But now, Cisco's policy-based ACI has overrun other SDN ideas at the company. The plan now is to apply policies from one controller, the APIC, to all manner of equipment. That would include optical gear, which isn't part of the APIC chain-of-command at the moment but could be included via OpFlex, the interface and client-side agent introduced by Cisco.

Cisco told Perrin that nLight still has a place in that strategy, as an intermediate step for packet-optical SDN. "Phase 2 is a multi-layer control plane network, enabled by Cisco’s nLight. Here, the wall between layers is opened up and multi-layer signaling is handled via a GMPLS UNI [user network interface]," Perrin wrote in a recent report. GMPLS is an established signaling standard that lets the packet and optical networks learn about one another's topologies.

Phase 3 would be to completely blend control of the packet and optical layers. At that point, "the SDN network is characterized by a centralized control plane, a global network view, and application driven actions (as opposed to human driven)," Perrin writes.

Cisco executives don't seem to have talked about OpFlex much at PONC, but they did establish one thing about their taste in SDN protocols: The HERO team will be favoring Netconf and YANG for network configuration, rather than OpenFlow, because that's what carriers prefer.

"The executives said that service providers are questioning whether OpenFlow can ultimately meet the five-nines requirements of operator networks [meaning 99.999 percent uptime], and, therefore, [are] favoring the IETF work," Perrin writes.

The Netconf/YANG strategy would seem to apply mostly to Phase 1 (which isn't mentioned above, but consists of today's separated packet and optical control planes) and Phase 2.