Mikrotik RB951g-2HnD

You can install it.
Load https://downloads.lede-project.org/snapshots/targets/ar71xx/mikrotik/lede-ar71xx-mikrotik-vmlinux-initramfs.elf and from it sysupgrade https://downloads.lede-project.org/snapshots/targets/ar71xx/mikrotik/lede-ar71xx-mikrotik-nand-large-squashfs-sysupgrade.bin

It should work.
You can check if switch works properly in initframs image without flashing

My RB951g is not accepting the initramfs.elf file. When I boot it with reset-buttom pressed, it is loaded from the tftp-server, but the unit boots into its "old" openwrt-image.

When I scp the sysupgrade.bin file to the unit running a newly compiled openwrt, and tries "sysupgrade /tmp/lede.....sysupgrade.bin" it (openwrt) answers:
Sysupgrade is not yet supported on rb-951g-2hnd.

Hm,that is weird.
Without serial you cant really see what happened

Any hints if I can build it myself with the patch

which works, when I build an openwrt-image.

Most likely that patch is already in LEDE

As far as I can see, there is only one Mikrotik target. The patch is needed on RB951g-2HnD but not on its 1. revision and not on RB951Ui.

(Btw. I have tried to install LEDE on a RB962 but gave up on it. Might be the same missing patch.)

In LEDE Mikrotik subtargets are separared by type of flash storage used.
64MB NAND,>=128MB NAND and 16MB SPI

So in your case you use NAND large

I can only see one initramfs.elf file. How can I get started if the initramfs-file is without that patch ?

Can I install NAND-large without sysupgrade ?

Yes,there is only initframs file used for all devices.
You ahould be able to use mtd to write the file to firmware partition

Can I get a little more help on that task. I am used to install openwrt on mtd, but there I have a detailed instruction to follow.
Which files replace

or how can I extract that content from the NAND-large sysupgrade.bin file ?

I dont know about installing OpenWRT but mtd writing nand large sysuprade image to firmware partition should work

Long ago I found this instruction to install openwrt:

root@OpenWrt:/# mtd erase /dev/mtd1
root@OpenWrt:/# mtd erase /dev/mtd2
root@OpenWrt:/# mkdir /mnt/kernel
root@OpenWrt:/# mkdir /mnt/rootfs
root@OpenWrt:/# mount /dev/mtdblock1 /mnt/kernel
root@OpenWrt:/# mount /dev/mtdblock2 /mnt/rootfs
root@OpenWrt:/# mv /tmp/openwrt-ar71xx-mikrotik-vmlinux-lzma.elf /mnt/kernel/kernel
root@OpenWrt:/# chmod +x /mnt/kernel/kernel
root@OpenWrt:/# umount /mnt/kernel
root@OpenWrt:/# cd /mnt/rootfs
root@OpenWrt:/# tar -xvzf /tmp/openwrt-ar71xx-mikrotik-DefaultNoWifi-rootfs.tar.gz
root@OpenWrt:/# cd /
root@OpenWrt:/# umount /mnt/rootfs

I don't know exactly what I am doing, but it works. Can anyone be more specific on, what to do with the sysupgrade.bin image ?

Can you at least check what do your partitions look like with cat /proc/mtd ?

Back from sleep and work.

root@OpenWrt:~# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00040000 00020000 "booter"
mtd1: 003c0000 00020000 "kernel"
mtd2: 07c00000 00020000 "rootfs"

So it is different than LEDE one,LEDE has firmware partition that contains both kernel and rootfs.
That is why sysupgrade does not work

Is that partitioning coming with the initramfs-boot from tftp, or is it from the original Mikrotik RouterOS?

Can be both.
But if you would be able to TFTP boot initframs image then sysupgrade would take care of partitioning

I can tftp-boot it with initramfs from one of my openwrt-builds.

Can I somehow extract the sysupgrade-exe from the NAND-large-sysupgrade.bin and use that exe-file to run the sysupgrade ?

You cant do that,initframs image contain the same stuff as sysupgrade images but are modified in a way that they are suitable for loading into RAM and running there.
To me it is really weird that you cant TFTP boot current LEDE initframs image

If LEDE has just 1 initramfs.elf file, there is a great chance it is without the patch. That means, it can be used on RB951U and other devices with a 100 M switch or an old 1000 M switch, but not on new versions of RB951G or RB962.

Anybody knowing how to extract the sysupgrade-exe from the NAND-large-sysupgrade.bin?