Basic certificate checking does not require an identity source. If you want binary comparison checking for the certificates, you must select an identity source. If you select Active Directory as an identity source, subject and common name and subject alternative name (all values) can be used to look up a user. https://www.cisco.com/c/en/us/td/docs/security/ise/1-3/admin_guide/b_ise_admin_guide_13/ b_ise_admin_guide_sample_chapter_01110.html