rlm_sql variable substitution clarification

I have run into an issue where we now have different types of NAS servers. I would like to use %{Connect-Info} if available or a string formatted from two attributes like : D%{Ascend-Data-Rate}_X%{Ascend-Xmit-Rate} This is how I tried to do it : ConnectInfo_stop = \ '%{Connect-Info:-D%{Ascend-Data-Rate}_X%{Ascend-Xmit-Rate}}' This is what I get when %{Connect-Info} is not available : "D_X" I haven't seen any examples where two attributes are combined to make one attribute. Thanks