Cisco doesn't recommend copying startup configuration files from NVRAM to an active configuration in RAM. They claim the files "merge" and the start doesn't replace the running file. This is assuming the router has already booted/loaded and you are playing around with the configurations. They recommend reloading the router. They say the copy command doesn't erase the active config and replace it with the startup config.
My question is why? Most of the time you use the copy command it erases the preexisiting desitination config file.
You can certainly copy from nvram to running-config and it can be quite useful, but it doesn't work the same as copying in the other direction. The simplest way to understand it is that it isn't really a "copy" at all. When you copy to nvram or flash, you're creating a new file and writing content to it. When you're done, the file only contains what you just wrote to it. Thus, if you did "copy run start", you have a complete copy of your config, and nothing more.
When you "copy" to running-config, this isn't really a file-creation operation. Instead, you're pushing text from the source file into the front-end parser of the running system, one line at a time, JUST AS IF YOU'D TYPED IT IN AT CONFIG T. Everything that's copied from the file becomes part of the running config (as long as it doesn't contain errors), but the running config may still contain many items that were already in it before the "copy" began--anything that wasn't explicitly changed by the text copied in. This is what Cisco means when they say it is "merged".
One handy use for copying from nvram to running-config is for making major structural changes to the router interface or switchport that you're talking on. If you were to key the commands in one line at a time with "config t", you'd likely reach a point where you lose connectivity, the next line is never received, and you're stuck making a field trip to finish the job. Instead, you can place your entire change in a little file "changes.txt", put a copy of it on your tftp server, and "copy tftp nvram:" to get it onto your device. Then "copy nvram:changes.txt running-config" to apply all the changes at once, without getting interrupted in the middle.
You can also use "copy startup-config running-config" to load up the config on a router that you initially booted with the config register set to skip the config. You might need to do this if you lost the enable password. Remember, however, that you'll need to go thru and fix all your interfaces, because Cisco defaults "shutdown" differently during the initial load than during subsequent operations.
Thanks for that. Not one I have noticed. I did consider recently tftping the whole startup off and on then "copy start run" to get just this effect but I decided it was too scary.
Also - on many (most now) platforms you can "copy tftp flash:" to get it onto your device. Then "copy flash:changes.txt running-config"
Hmm. I had always wondered what the nvram: file system was for.
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