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So let’s have a look at a difference in configuration.

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So what happens if I type sh run | b vty.

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If I look at the startup config  I have the same configuration

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but let’s change that so, line vty 0 10 login password cisco

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sh run | b vty I can see my 5 VTY lines

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0 4 plus an additional 6 VTY lines.

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If I type sh start | b vty

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I only see my first 5 VTY lines with the command login.

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So let’s copy start to run and see how it merges the configuration.

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Well it removes config, overwrite it or replace it.

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So sh run | b vty we still have the VTY config as configured.

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So the login command is still there, the password command is still there

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and the additional VTY lines are still there.

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Sh start | b vty, only has the first 4 VTY.

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But now let’s make a change, lines line vty 0 10

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I’m gonna type no login, so sh run type begin vty notice we’ve use the command no login

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which means no login credentials are required, but if we look at the startup config

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we’ve got the login keyword, so this login keyword conflicts with this no login command.

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copy, copy start run   sh run | b vty

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so notice the difference now.

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On the VTY lines, this command has been overwritten by the startup config

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because previously in the running config

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we had no login and then the startup config we had login.

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So no login was in conflict with login and that’s now been replaced

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but the additional conflict hasn’t been removed.

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So the startup config has been merged with the running config.

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Be careful doing merges of config, it can be confusing

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to know what your result in configuration is.

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Now there’s another option called configure replace.

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So now let see what happens if we replace, the config with the startup config.

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So I'm gonna replace the running config with the startup config

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notice we’ve told that configuration changes have been made.

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So if I type sh run | b vty

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the default login authentication has been added.

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the configuration of all the lines have been replaced with the startup config.

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So this is not a merge, this is the replacement of the running config with the startup config.

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So just to reiterate, copy start run is a merge

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configure replace startup config.

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configure replace nvram startup config is a replacement

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of the running config with the startup config.

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So be careful using copy start to run

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because the result in configuration may not be what you expect it to be.

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A copy runs start is a replacement always but a copy start to run is a merge

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so a command that often preferred is configure replace

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which replaces the running config with the startup config

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the same could be used, so it configure replace

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and you could  specify TFTP rather options

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so you could replace the running config with the configuration on a TFTP server.

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By default a copy from TFTP to run is a merge

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in the similar way to when you make a configuration changes through the CLI

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like we’ve been doing you are merging the configuration

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in other words you're making changes to specific portions and the same is true

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when you copy the config from startup to running config, be aware of the different options.