Every Cisco device has 2 sets of configurations. On routers and switches these configurations are called ‘running-config’ and ‘startup-config’ When you make any changed on a router or switch, the change applies to the running-configuration and will affect the router or switch with immediate effect. However, if the device is reloaded for whatever reason, all your changes will be lost as they are not in the startup-config. In order for you to save your configuration, the contents of the running-config needs to be copied to the startup-config (which the router or switch uses at boot). There are 2 ways to save. You can use the ‘copy running-config startup-config’ command, which can be shortened as ‘copy run start’ or you can use the ‘write’ command, shortened as ‘wr’.
NOTE: If you are planning on sitting any Cisco exams, the simulators in the exam DO NOT support ‘wr’ and you have to use ‘copy run start’
In the live demo below, I have changed the hostname of my router from ‘AOIP.ORG’ to ‘AOIP’, you will notice when I looked at the running-config the change has happened, however when I looked at the startup-config, it is still set as ‘AOIP.ORG’. Only after I have saved my configuration does the change reflect in the startup-config.
NOTE: Save soon, and save often!