Which two statements are true about IS-IS? (Choose two.)

A.
All level 2 PDUs are flooded to Level 1 by default.

B.
All level 1 PDUs are flooded to Level 2 by default.

C.
Both Level 1 and Level 2 adjacencies can exist on the same interface.

D.
Level 2 external routes are always available in Level 1.

Explanation:
Link-state protocols use a different mechanism to determine the best route. Two of the newer routing protocols, Open Shortest Path First (OSPF) and Intermediate system to intermediate system (IS-IS), use this mechanism. These protocols send out special packets OSPF calls them link-state advertisements (LSAs) and IS-IS calls them link-state protocol data units, or link-state PDUs (LSPs) to find out who their neighboring routers are and what speed the connections are between routers. From this information, the routing protocols create a database that defines the network topology. They then run the Dijkstra shortest-path first (SPF) algorithm on the information in the database to determine the shortest path to each destination in the network.

Once the LDP session is established, the routers maintain adjacencies much in the same way that OSPF routers maintain adjacencies, and when the topology changes, those changes generate LDP messages that allow LDP to set up new paths.

Link-state PDU – Packet that contains information about the state of adjacencies to neighboring systems.