If you have upgraded your router from any other IOS release to release 12.4T without changing the NAT configuration (or used NAT configuration known to work into a router running IOS release 12.4T), you might have encountered weird behavior due to the changes in NAT implementation. The unexpected behavior and configuration fixes needed to avoid the NAT-related problems are described in my new NAT Caveats in IOS release 12.4T article.
Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.