[ns] How to shut down a link or node

 I am going to make a link (or at least a node) go down at runtimewithin a procedure:

$ns rtmodel-at $now down $n0 $n1

Actually the code is executing, but nothing seems to happen and no error will be produced!!!
I am not sure whether the problem is due to calling the rtmodel inside a procedure.

Please advise if there is other way to shutdown a link (node) in Ns2.

Thank you in advance,

Regards,
Hassan