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.

Explanation:

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.0/routing/configuration/guide/rc3isis.html#wp1148617
Single-Topology IPv6 Support
Single-topology IPv6 support on Cisco IOS XR software allows IS-IS for IPv6 to be configured on
interfaces along with an IPv4 network protocol. All interfaces must be configured with the identical
set of network protocols and all routers in the IS-IS area (for Level 1 routing) or the domain (for
Level 2 routing) must support the identical set of network layer protocols on all interfaces.
When single-topology support for IPv6 is used, only narrow link metrics, also known as old-style
type, length, value (TLV) arguments, may be employed. During single-topology operation, one
shortest path first (SPF) computation per level is used to compute both IPv4 and IPv6 routes.
Using a single SPF is possible because both IPv4 IS-IS and IPv6 IS-IS routing protocols share a
common link topology.
Because multitopology is the default behavior in the software, you must explicitly configure IPv6 to
use the same topology as IPv4 in order to enable single-topology IPv6. Configure the singletopology command in ipv6 address family configuration submode of the IS-IS router stanza.
Multitopology IPv6 Support
Multitopology IPv6 support on Cisco IOS XR software differs from Cisco IOS software in that IS-IS
assumes that multitopology support is required as soon as it detects interfaces configured for both
IPv6 and IPv4 within the IS-IS stanza.
You must use the metric-style wide command to configure IS-IS to wide link metrics as
multitopology link advertisements.