aaa new-model
aaa authentication login default group tacacs+ enable
aaa accounting exec default start-stop group tacacs+
aaa accounting commands 15 default stop-only group tacacs+
aaa accounting network default start-stop group tacacs+
aaa accounting connection default start-stop group tacacs+
aaa accounting system default start-stop group tacacs+

tacacs-server host 10.100.100.100 timeout 5
no tacacs-server directed-request
tacacs-server key 7 key

Also check :

1. Cisco OSPF over GRE tunnel