Implications of Reference System in SAP Portal

Hello Friends,

We have a situation where 30 of our existing users have IDs exceeding 12 characters in current non-SAP system. In ECC 6.0, I believe there is a limit of 12 characters for user IDs.

As these users would be using SAP portal as their primary means of interaction with all SAP back-end systems, we are planning to keep their existing user IDs in SAP portal and creating another user ID (<12 chars) in all back-end systems (ECC/BW/CRM). We'd use ECC system as a reference system to map the userIDs.

I've a few questions regarding this approach.

-Is user mapping necessary for all portal users or only these 30 users need to maintain it?

-Is it mandatory for the reference system to be up and running all times? Should the reference system breaks down for some reason, would it hamper connectivity of all mapped users to other systems as well?

Appreciate your inputs.

Thanks,

Vishwas Madhuvarshi