O que você está procurando é AAA (Autenticação, Autorização e Contabilidade), no site de suporte da Cisco:
"AAA enables the security appliance to determine who the user is (authentication), what the user can do (authorization), and what the user did (accounting). AAA provides an extra level of protection and control for user access than using ACLs alone. For example, you can create an ACL allowing all outside users to access Telnet on a server on the DMZ network. If you want only some users to access the server and you might not always know IP addresses of these users, you can enable AAA to allow only authenticated and/or authorized users to make it through the security appliance. (The Telnet server enforces authentication, too; the security appliance prevents unauthorized users from attempting to access the server.) You can use authentication alone or with authorization and accounting. Authorization always requires a user to be authenticated first. You can use accounting alone, or with authentication and authorization. This section includes the following topics:
•About Authentication
•About Authorization
•About Accounting
Mas, até onde eu saiba, você precisará configurar um "Servidor Cisco Secure Access Control". Acho que seria melhor se você usasse uma solução de filtragem da Web (websense e similares). você também pode configurar um servidor squid, integrado com o ldap e configurar o ACL com base no usuário autenticado.