Sometimes, when there are many network administrators happens that more than one is making changes into one router at the same time. This may generate really big problems due to overlapping configurations and that more than one config is being applied at the same time.

Cisco IOS provides a simple solution o this dual-head problem. We can lock the device allowing only single-user configuration.

Router(config)#configuration mode exclusive ?  
 auto Lock configuration mode automatically  
 manual Lock configuration mode on-demand

First one will lock configuration mode whenever somebody enters the configure terminal command. Second one will lock configuration mode manually. I mean when somebody manually specifies that wants to lock the configuration mode. With both, if somebody tries to configure the device when its locked by another user, will get the following message:

Router#conf t  
 Configuration mode locked exclusively by user 'netop' process '187' from
terminal '7'. Please try later.