MTU Setting Too Low For Xbox Live at 1500?

I am having difficulty connecting to games over Xbox Live, and keep getting kicked offline. When I run a connection test on my Xbox, it tells me that the problem is that my MTU setting is too low (below a minimum of about 1300). However, when I go to my internet router's admin panel, it shows that my MTU setting is at 1500, well over the minimum setting. Does anybody know of a way around this problem, a different setting that I can change, etc?