EXHANGE SERVER (BINDING WITH RPC OVER IP

I AM HAVING PROBLEMS CONNECTING TWO EXCHANGE SITES. ONE HAS BEEN ESTABLISHED ON A PDC. THE OTHER IS A NEW INSTALLATION AND IT IS ON A BDC ON A DIFFERENT NETWORK. THE TWO ARE CONNECTED BY A PASSPORT 4460 ROUTER SETUP. I HAVE TESTED RPC COMMUNICATIONSAND THEY WORK FINE BUT EVERY TIME I TRY TO ADD THE OTHER TO THE SITE IT FAILS WITH AN ERROR SAYING RPC COMMUNICATIONS FAILED. WHAT COULD CAUSE THIS PROBLEM AND IS THERE A WORKAROUND SO THAT I MAY USE WINS OR DNS. IT IS A VERY FLAT NETWORK. WE HAD RECENTLY SEPERATED THE TWO NETWORKS ON 10.10.5.0 CLASS C AND 10.10.2.0 CLASS C. THE ROUTERS DO NOT FILTER ANYTHING OUT. HE USER NAMES ARE REPLICATED CORRECTLY ON BOTH SITES BUT EXCHANGE IS THE ONLY THING THAT WON'T TALK.