How do routers in an IS-IS Level-1 domain exit to reach other Level-1 （L1） domains？（）
- ALevel-1（L1） routers use default routes announced by Level-2（L2） routers in Level-1（L1） domain
- BLevel-1 routers use default routes installed based on ATT bit （Attach Bit） in announcements from Level-1-2（L1/L2） router
- CLevel-1 routers use specific routes， for other Level-1 domain， announced by Level-1-2（L1/L2） by route-leaking feature of Cisco IOS
- DLevel-1 routers use specific routes， for other Level-1 domain， announced by Level-2（L2） by route-leaking feature of Cisco IOS