I turned back to my WRT610Nv2. I need some experiments with ipv6 of my ISP. I used it only as wireless point connected to ZyXEL USG50 before that. The firmware is Tomato Shibby v132. Now I want to test it like a router. So I flashed it with stable build 17.01.4 of LEDE again, as I have done some month ago. Surprisingly I noticed that there is no WAN functions in my device. I have only LAN. When I plug network cable to WAN port, it works like LAN too. I have no VLAN config page in WEB interface too. It's very strange. When I flashed LEDE for the first time I had all these things working. Then I deleted WAN from config as I needed only wireless access point, not a router. After realizing that 5 GHz band doesn't work I returned to Tomato firmware. It seems that that old config stored somewhere in the router. But I have made hard-reset with reset button several times. Flushed DD-WRT and Tomato. They are working, no problems. The problem is only with LEDE. Where can I find default network config for my device to make switch work? I remember that there were eth0.1 and eth0.2 interfaces. Now I have only eth0.
See this section of the documenation...
Ok, thank you. I'll inspect this. And why the default config disappeared? What's wrong?
If you configured those after LEDE was installed, they will be reset when LEDE is flashed.
I've told already. I flashed LEDE over Tomato firmware. There were WAN,WAN6, LAN -settings. Also there was a switch configuring option in Network settings. These options were out of the box. I didn't configure them manually. At that moment I didn't need WAN. I deleted it. Later I went back to Tomato firmware. Now I flashed LEDE again, the same image, and there is no WAN and switch settings at all! I wonder, why they are disappeared?
I read what you said.
I can tell you that switch options exist in my /etc/config/network in the current stable release, but they are not configured with eth0.1 and eth0.2 out of the box.
You need to configure the switch.
Good reason to save your config file before flashing.
If you don't have the Network->Switch page, there is probably no switch driver in the build.
Did you flash the same version of LEDE you were using before?
There should be no memory of settings between the Tomato environment and LEDE.
Of course. I saved that image file on my computer. Also I downloaded this file again, and try it. There is no option to configure switch on this device now. All ports including WAN work like a LAN.
I don't remember the names of these interfaces. It was three months ago. I don't pay attention to the names. I remember that there was a switch option, where I can configure VLANs. Also there were WAN and WAN6.
I have no such config. I flashed LEDE, tried it, then went back to Tomato. Can you give me the default config for WRT610N network settings?
It's whatever is in /etc/config/network after you flash LEDE.
To see that, SSH in to the router and run cat /etc/config/network
You might want to review the OpenWrt/LEDE documentation...
OK, thank you. I know that. But in this config I can see only one interface. There is no WAN or so on. You mean that default behavior of my router is the only one LAN-interface? The first time I flashed LEDE threre were all interfaces (WAN and LAN). Where are they now? It looks like LEDE stores some settings in the area of flash memory that doesn't erase by hard-reset.
Did you get the stable build here?
https://openwrt.org/toh/views/toh_fwdownload?dataflt[Model*~]=wrt610&dataflt[Versions*~]=v2
Yes. I downloaded image from this link. I flashed it without problems some month ago. And now using the same image I have no WAN interface. May be some problem with my router? But it works fine on Tomato and DD-WRT. May be there is some special erase procedure to flash LEDE?
No special erase procedure, except what is listed in the documentation...
https://openwrt.org/docs/guide-user/troubleshooting/failsafe_and_factory_reset#factory_reset
Failing that, if it were my router, I would flash back to stock Linksys firmware, reset that to factory, then flash OpenWrt/LEDE.
May be I try to go back to stock firmware. But it's very strange.
Thank you! You are guru! 
I flashed stock Linksys firmaware, and after that flashed LEDE release. Now I can see WAN, WAN6 interfaces and switch settings! It's really fantastic!
Amazing, but... does anyone have any explanation for this behavior? It seems as if some record was not erased from some memory pointer. Is this possible? It is cristal clear that the configuration files are overwritten with each firmware change, so... Is it possible to activate or deactivate the ports outside the configuration files?
I think so too. But I can't believe that it's possible.