How do routers in an IS-IS Level-1 domain exit to reach other Level-1 （L1） domains？（）

• 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