I have always wondered about this but never asked. I have the MD-11, NGX, and 747-400. By default the 747-400 shows all the traffic no matter what the altitude. In the NGX you can choose this as an option or you can use filter. But what about the MD-11 is there any way to display all traffic? It displays traffic that is within maybe 2500 feet above or below you but thats it. Never saw any options anywhere to change it.

Try pressing the TFC/TRFC (whichever one it is. I can't remember which...) button on the ND control panel. Other than that I think it's hard coded (I think that's the correct term) to 2500/1000 feet.