In previous posts we saw how the single-lambda 100G approach to optics helps simplify designs and enables massive volume manufacturing. This has come to fruition in the QSFP28 100G FR pluggable transceiver.

Let’s also consider its relation to the latest generation of switches and routers with 400G ports. Single-lambda 100G is used in 400G optics technology as well, and because of that, it allows for network upgrades with the greatest operational flexibility. The key is that it enables 400G optics to interoperate with 100G optics via fiber optic breakout. For example, 400GBASE-DR4 modules in a QSFP-DD form factor, such as Cisco’s QDD-400G-DR4-S, can connect with four separate QSFP28 100G FR modules. This means that if your network uses host platforms with 100G QSFP28 ports filled with FR optics, you can upgrade just one site with 400G host platforms and still connect it to the existing 100G sites without sacrificing port bandwidth.

Let’s say you had ASR9k routers with 100G ports at two nodes of your network (see figure below). In one node (on the left), you could replace it with the 8000 Series router with 400G QSFP-DD ports, and then connect it back to the 100G ports in the ASR9k at the other end (on the right). You would use 400G QSFP-DD DR4 modules in breakout mode, and connect each DR4 to four individual 100G QSFP28 FR modules with the help of a fiber breakout cable or cartridge.

How does this help? It minimizes your network downtime. With this interoperability between your legacy 100G host and your new 400G host, you can upgrade one site at a time, instead of bringing down the entire network to upgrade all sites simultaneously. Doing everything all at once is likely to be prohibitively expensive and infeasible in practice.

At any given time, you could have a mix of new 400G sites and legacy 100G sites, still operating and connected to each other. The example in the figure below shows the various connections between different pairs of sites.


Even though we are committed to optics, we know that the bottom line is how the optics impact the network performance and deployment process. This is yet another way that Cisco provides benefits of optics at the network level.

Look for more blog posts in the upcoming weeks on single-lambda 100G. Better yet, subscribe to the Cisco Optics blog!

And don’t forget to view the Cisco Optics Youtube playlist.