Quang

The title of your post is about erase username and password, but the text of your post indicates that you have forgotten the name and password. What you need to do is to execute the password recovery feature. The basics of the password recovery procedure are that you set the config register to 0x2142 so that the router bypasses the config file. Then you can get into enable mode, you copy the config from startup-config to running-config, and then you can recreate the username with a password that you will remember. Be careful to set the config register back to its original value when you are done.

HTH

Rick