A.Level-1（L1） routers use default routes announced by Level-2（L2） routers in Level-1（L1） domain
B.Level-1 routers use default routes installed based on ATT bit （Attach Bit） in announcements from Level-1-2（L1/L2） router
C.Level-1 routers use specific routes， for other Level-1 domain， announced by Level-1-2（L1/L2） by route-leaking feature of Cisco IOS
D.Level-1 routers use specific routes， for other Level-1 domain， announced by Level-2（L2） by route-leaking feature of Cisco IOS

A.By default，Netflow records bidirectional IP traffic flow.
B.Netflow answers questions regarding IP traffic：who，what，where，when，and how
C.Netflow accounts for both transit traffic and traffic destined for the router.
D.Netflow returns the subinterface information in the flow records.