I just ordered an Asus RT-N18U and I would like to install Tomato Firmware to this router.
I am interested in this Tomato, because I see many possibilities in this.
The only thing that aim afraid of is, what if I mess up something at the firmware installation? Can I install back the factory default firmware?
I've already checked many tutorials and videos on YouTube, but they were tutorials for another Asus RT-Nxx router.
Asus routers are very resilient when it comes to putting third-party firmware on them. Most if not all Asus routers use a boot loader known as CFE which stands for common firmware environment and is essentially the bootloader for the router.
The CFE has a recovery mode that when triggered accepts a new firmware file to be flashed over the existing firmware in the event that it doesn't boot properly (which is known as soft brick) and is usually triggered by holding down a button (varies by router but usually it's labeled reset or in some bizarre cases WPS) when the router is powered on.
Good news it's hard to break the CFE.
As long as you have the files in hand before you start to flash the router you should be fine just remember that if you are relying on the router for internet access you won't be able to fix it in the event you have the wrong or no files to do so.
It is highly recommended that you have the latest official firmware file on hand in case of a bad flash.
Assuming that you follow the procedure perfectly, including correct choice of firmware version for your router model and hardware revision, you shouldn't brick it. It doesn't mean that firmware will work flawlessly - it may contain bugs - but it should flash and work as well as it can.
If something goes wrong during the procedure (for example you fail to erase NVRAM correctly) you may encounter some bugs. They can be severe or unimportant, and in some cases may render device unusable. You may be able to fix these by following the procedure again if you're able to start flashing (bugs may prevent it). It's even possible that device won't boot correctly.
The firmware may also contain bugs that break some features or make them hard to use, for example it may lack 5 GHz (802.11n) support or flashing may not work (ie. you won't be able to flash another/original firmware easily).
What happens when flashing goes wrong and you can't start flashing again? Some routers and firmwares feature "emergency flashing" mode. To use this mode you have to connect router directly to a computer with a cable, set specific static IP on the PC, and transmit new firmware using some trivial (and probably not user-friendly) protocol (telnet, TFTP etc.)
If emergency flashing doesn't work, it's sometimes possible to flash firmware directly into memory. Some advanced hardware and knowledge is needed to do this and if you're in this situation, you may want to visit nearby hackerspace and ask for their help.
Some useful info: The Peacock Thread.
