Hi Fabrizio,
The answers to your questions about discovering the F5 Device (BIG-IP) in SCOM are:
1) Could I do it like BIGIP is a UNIX/Linux device?
I would recommend installing the F5 Management Pack (F5 MP) and perform the F5 Device discovery from within the discovery wizard installed by the F5 MP in SCOM (and not the [native] SCOM discovery wizard). The F5 MP discovery provides full configuration discovery and statistics-collection functionality for the BIG-IP. In contrast, if you use the default SCOM discovery wizard, for discovering the BIG-IP as a Unix/Linux device, you'd only get a very limited config/stat discovery for the F5 Device, as a basic Unix/Linux platform. And we don't support this type of discovery. Nor do we support the discovery of the BIG-IP through SNMP (in case you'd thought about this alternative as well).
2) Should I must insert root account credentials (root/default) to perform a discovery?
No, you don't need root account credentials to the BIG-IP for discovering with the F5 MP. The account needs to have enough privileges to perform an iControl connection to the F5 device and also to upgrade the big3d agent, if necessary.
3) Could I use a custom user for discovery it? (f.i. UserSCOM)
When you first discover the F5 Device within the F5 MP discovery wizard you will be prompted for the BIG-IP user credentials, which will be cached within a secure token (per device) on the local system, for further (re)discovery and device connectivity. Subsequently, you can re-discover the F5 device, which is performed through an F5/SCOM task, running either as the default SCOM action account or a custom account, that you can specifiy in the task's 'Run as' option.
Let me know if these answers are good enough for you, or if you have any other questions.
Thank you,
Julian