cisco interface command serial interface goes down frequentl

hi all . i am preparing for ccna ,and using boson net sim version 5.27 for practice lab.when i try to configure serial interface 0 on any of the lab and assign an ip address it accepts it problem comes when i give the no shut command to activate the interface it goes up and after one ro 2 sec it shows the following message

%lineproto-5-updown line protocol on iterface serial0 changed stte to down
i m using cisco 2500 router simulators
any idea what should i do i am stuck up as i cant proceed with my labs :roll:

thanks
ali anwar[/b][/size][/color][/color][/color]

I can't say for sure as far as the simulator goes, because I've never used one, but on a real router, the interface actually needs to be connected to something on the other end for it to actually show up/up (i.e. connected to a CSU/DSU, or connected to another router via a serial cable).

You could use the 'no keepalives' command for that interface. This will prevent keepalive messages from being sent and checked, and the interface will remain in the 'up' status.

You should also ensure that the clockrate is set on the DCE side of the connection and that the serial interface at the other end of the connection has also been configured properly and given the no shutdown command.