Trying to install OpenWrt on a Velocloud 520-AC

That's what I referred to with 'possible, but a heck of a lot of work', sources are available, but they are very invasively changing igb (with the necessary MDIO communication channel for the marvell switch). Getting this extracted from the ancient kernel sources and then forward ported to OpenWrt's modern kernel will be... difficult... and time intensive.... and given the effort, it would have to get merged mainline first, which makes it yet a notch more difficult.

It's a 'possible' task for someone experienced, but expect it to take months+ of rather constant development and bartering with Intel, Marvell and linux-netdev, assuming you find this hypothetical someone in the first place.

it's really too bad because it's not worth anything anymore

That's actually a really good price too. I might buy this just because it's so cheap lol. I mean like I said, if you're willing to use the SFP ports with rj45 adapters it's really not a bad little router.

Also the wifi hardware in these is pretty robust considering this isn't really meant to be an AP.

Hi , i've got two doorstop's 510 ac lte and 520 , but i can't get to boot menu with f12 or del, what i can do ?

Keep using them as door stops.

yeah, not so opensource friendly device, i'll play with them lil bit more, got it openwrt loaded but watchdog is annoying bit. any hint how to get rid of it?

It's all in this thread...

This device is not a no-go area, but getting it supported would involve quite considerable patches to be (adapted to- and) merged mainline via the netdev developers. It should be possible, but will be far from easy - both to get it working and to beat the patches into a shape to become acceptable by Intel's- and the mainline kernel developers.

If you approach this topic from an enduser's perspective, chances are that it's never going to be supported. The hardware is very special (one-off for this vendor/ model range), the drivers are hard to find and 'ugly'. The chances that someone with the necessary developer skills buys this hardware and spends their time on it to get it mainline are slim to none.
If you want to retain your sanity, skip them for supported/ easier kit (with an individual network chip per port, not an onboard switch with complicated MDIO communication hooking into the Intel network driver).

If you approach it from a developer's perspective, it's going to be a challenge - possible, but a lot of work. These devices are cool, they could be a great addition for OpenWrt and similar uses, but it is going to be difficult and will take months of your time.

This will be an increasingly common problem in the future. I have noticed EOL/EOS business-grade x86 firewall appliances, previously a great and plentiful source for powerful OpenWrt routers, moving from individual ports to integrated switches, especially if more than four or five ports are involved. The Sophos XGS136 is a prime example, or more obscurely the TrendMicro Cloud Edge G2 or Saxa SS5000 series.

Increasingly common perhaps, but sadly I do assume that each of those has its own custom hooks into e1000/ e1000e/ igb/ igc - so probably not any more likely to be supported.

But, again, it's very much possible - 'just' a heck of a lot of work.