For dense deployment and limited faceplate
space, 100g-fr / dr are the only way.
LR4 is dead to me.
We run the SR4 optics for in-rack cabling, because they are about 4X
cheaper than all the single-mode options.
We have been heavy on the LR4 optics, but are now starting to test
(and if happy, switch to) the FR units, as they are even cheaper
than the LR4 option.
The DR options don't make much sense for us, because we prefer the
SR4 for in-rack cabling, and given how large data centres are
nowadays, 500m might not enough to interconnect with customers.
But for un-amplified Metro-E deployments, we are looking forward to
testing Adva's 100G-ZR, which is also a single lane optic.