And to expand on that slightly, it is not just about density, but also latency. Since 40G is clocked the same as 10G (as it is 4x 10G lanes), upgrading from 1

And to expand on that slightly, it is not just about density, but also latency. Since 40G is clocked the same as 10G (as it is 4x 10G lanes), upgrading from 10G infrastructure to 40G will not improve overall latency Whereas 25G is clocked faster and will have lower overall latency. [Overall meaning the first packet won't arrive relatively sooner, but the second one will.] There may be latency improvements inherent from design improvements within newer packet processing / switching ASICs themselves, just like Intel's tick/tock. For example, NASDAQ offers a 10G handoff to their 40G infrastructure which is 5-9us faster because of topology and equipment upgrades [1]. The nice thing about 40G is that it is compatible with 10G so you can selectively upgrade your switches to 40G but keep your 10G NICs, using these neato QSFP+->SFP+ breakout cables [2]. [1] http://www.nasdaqtrader.com/content/Productsservices/trading/CoLo/LowLatencyFS.pdf http://www.nasdaqtrader.com/content/Productsservices/trading... [2] https://www.google.com/search?q=qsfp+breakout&tbm=isch https://www.google.com/search?q=qsfp+breakout&tbm=isch