Setting up Bank Codes for the same bank but with multiple DFI ID's

We are implementing Cash Management in 9.2 but we are struggling with understanding the proper setup for banks and external accounts.  Our bank accounts fall under one of three ABA numbers: 011900571, 053000196 and 121000358.  These are all BOA.  We created one Bank Code - BOA and assigned it DFI ID 011900571.  We then added every account using Bank Code BOA but assigned each account number its correct corresponding ABA number.  However, when we attempted to import our BAI2 file a number of bank accounts were rejected:

Account 0000000##### does not exist for Bank 011900571.

This message seems to indicate we need to create a Bank Code for each ABA number and that each bank account assigned to a given bank code should have the same ABA number as the bank code.  This seems to render the ABA number on the bank account redundant.  Is this in fact the correct approach?  If so, why have the DFI ID at the bank account level.