Which four statements are correct regarding IS-IS operations?

Which four statements are correct regarding IS-IS operations? (Choose four.)
A. By default, Level 1 routers within an IS-IS area do not carry any routing information external to the area to which they belong. They use a default route to exit the area.
B. Summarization should be configured on the Level 2 routers, which injects the Level 2 routes into Level 1.
C. IS-IS supports “route leaking” in which selected Level 2 routes can be advertised by a Level 1/Level 2 router into Level 1.
D. The IS-IS backbone is a contiguous collection of Level 1 capable routers, each of which can be in a different area.
E. With IS-IS, an individual router is in only one area, and the border between areas is on the link that connects two routers that are in different areas.
F. Cisco IOS XR Software supports multitopology for IPv6 IS-IS unless single topology is explicitly configured in IPv6 address-family configuration mode.