Many people are noticing that part of the Cisco ACI announcement includes mention of 40GbE BiDi optics where a 40GbE SFP needs only two MMF fibre cores for 40GbE connectivity. It’s worth noting that all vendors will also ship BiDi optics in the near future. For example, Arista is promoting a single mode 40G BiDi that uses a single mode pair to achieve less cabling while Cisco has chosen to lead with MMF version to fit it’s customer base better.
This isn’t innovation. Someone else made these optics and manufactured them while Cisco & Arista are simply selling them on now that they have put support in their software. They won’t be available for a couple of months anyway and, no doubt, they will be reassuringly expensive.
The difference is simple – doing BiDir on MulitMode Fibre is good for existing data centres where someone has overspent/overcapitalised on cabling infrastructure already by flooding the data centre racks with cables.
Single Mode Fibre is better for new build since SMF has a 20 year future while MMF is much less likely to be useful in 20 years and works much better with modular cabling systems.
Consider the wider situation: a single MPO patch cable up to 1000m with 24-cores of SMF can now connect 12 x 40GbE ports in the network backbone. You can install this type of cabling “just in time” and “as needed” instead of wasting large amounts of money on the “standard practice” of cabling the entire data centre with unused (and often unusable) cabling.