Okay, so I sit my 640-801 soon and am fairly confident with probably 85 - 90% of the material. Sure there are a few bits and bobs I need to brush up on but there is only one topic I have covered in little detail if any

.....that's ISDN! .....I keep putting it off but not got much time to put it off any longer!
Does anyone have a suggestion on how to go about simulating an ISDN lab between two 1721 routers, to act as a backup link on DDR for say if there was a "fault" with my serial interface so communication could continue?
I have entered the ISDN commands into router simulators and the lab activities in the curriculum but I would ideally like to simulate this in my lab for peace of mind if anything and feel doing this will give me a better understanding.
I guess the first place to start is what type of cable required to link between the two routers crossover or straight or doesn't it matter?
I'm also a bit confused as to what the spid would be or would I just make one up?
Thanks