There is provided an intra-realm AAA (authentication, authorization and accounting) fallback mechanism, wherein the single global realm may be divided in one or more sub-realms. The thus presented mechanism exemplarily comprises detecting a failure of an authentication server serving at least one authentication client within a first sub-realm of a single-realm authentication system, and routing authentication messages of the at least one authentication client to a fallback authentication server within a second sub-realm of the single-realm authentication system, wherein routing may exemplarily comprise sub-realm based source routing.