Cisco revamps IP over DWDM with Routed Optical Networking

Cisco (NASDAQ: CSCO) used the 2021 edition of its annual Cisco Live! event to unveil Routed Optical Networking, a new router-based network architecture that sees optics integrated into routers to simplify the network, save costs, and enable better IP and optical layer integration. If this sounds familiar, it should: Cisco has been touting IP over DWDM, a concept said to create similar benefits, since the early part of this century without significant traction. However, advances in both router and optical technology now make the integration of optics and routing more efficient and economical, according to a Cisco source, which means that network operators should find Routed Optical Networking more appealing than its predecessor.

IP over DWDM sought to remove the need for standalone optical transponders by integrating transponder functions into the router. Cisco and other router manufacturers developed DWDM cards that could be integrated into one (or more) of the router’s slots. However, there were several drawbacks that became apparent in implementation that limited the appeal of the approach according to the senior vice president and general manager of Cisco’s Optical Systems & Optics Business Unit, Bill Gartner:

• The DWDM line card robbed routers of at least one slot. So router capacity decreased.
• While the use of the DWDM line card removed the cost of a transponder and the associated short-reach optics, the card itself was fairly expensive. So the capex economics weren’t as compelling as hoped.
• Routers have significantly shorter lifecycles and upgrade times than DWDM optics. With DWDM line cards having to be replaced with the advent of new routers, the total cost of ownership economics weren’t as compelling as expected either.
• Some network operators had separate groups responsible for the optical and IP layers, which presented organizational challenges in implementing IP over DWDM in some cases.

Gartner says this last issue is not as pronounced now as it has been in the past. Meanwhile, a pair of technology advances have solved the problems just mentioned. The first of these is the advent of coherent pluggable modules, particularly 400ZR/ZR+ devices, in formats such as QSFP-DD. Since these transceivers can be plugged directly into router ports, no router slots need to be devoted to optics and optical line cards are no longer needed. And while there have been pluggable coherent modules available previously, such as CFP2-DCO, they have been larger than client-side port form factors, which meant that they still carried a router capacity penalty because router cards would support fewer ports. As 400 Gigabit Ethernet also will use QSFP-DD (as well as OSFP), the coherent and non-coherent form factors finally match.

Second, router ASICs have grown increasingly powerful, to the point where they offer more performance headroom than did previous generations. Thus, the useful life of routers has grown nearer to that of a typical optical system. The result, says Gartner, is that operators can now benefit from integrated optics and router systems and safely rely on router chains – rather than bypass them – when architecting their networks. And early cost modeling has suggested the resultant opex savings can be as great as 45%, according to Cisco’s website

When fully implemented, IP and private-line services are merged onto a single IP/MPLS layer and take advantage of the potential for network automation in a fully open, flattened yet massively scalable architecture. However, Gartner allows that while SDN and related software-driven network automation approaches are of interest to network operators, the journey toward such a nirvana will take different paths for different operators. Meanwhile, few will want to cast aside optical network hardware that’s working well in the field. Gartner thus sees the adoption of Routed Optical Networking as an evolution for most, one that likely will start with the adoption of pluggable optics. As routers are replaced (Cisco has capacity increases in the works for such workhorses as ASR 9000, the 8000 Series, and the NCS 5500), some of the other benefits of Routed Optical Networking will be more easily accessible, Garnter concludes.

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