link avoidance

a fellow researcher wants > to make the case that in some scenarios it is very important for a > network operator to be able to specify that traffic should *not* > traverse a certain switch/link/group of switches/group of links > (that's true right?). Could you give some examples? Perhaps point > me to relevant references? if so, why? security? congestion? other? but is it common? and, if so, how do you do it? randy