What are three characteristics of the OSPF routing protocol? (Choose three.)

A.
It converges quickly.

B.
OSPF is a classful routing protocol.

C.
it uses cost to determine the best route.

D.
It uses the DUAL algorithm to determine the best route.

E.
OSPF routers send the complete routing table to all directly attached routers.

F.
OSPF routers discover neighbors before exchanging routing information.

Explanation:
OSPF builds routing tables based solely on the destination IP address found in IP packets. It was designed to support variable-length subnet masking (VLSM, CIDR). OSPF detects changes in the topology, such as link failures, very quickly and converges on a new loop-free routing structure within seconds. For this, each OSPF router collects link-state information to construct the entire network topology of so-called areas from which it computes the shortest path tree for each route using a method based on Dijkstra’s algorithm. The link-state information is maintained on each router as a link-state database (LSDB) which is a tree-image of the network topology. Identical copies of the LSDB are periodically updated through flooding on all routers in each OSPF-aware area (region of the network included in an OSPF area type – see "Area types" below). By convention, area 0 represents the core or "backbone" region of an OSPF-enabled network, and other OSPF area numbers may be designated to serve other regions of an enterprise (large, business) network – however every additional OSPF area must have a direct or virtual connection to the backbone OSPF area. The backbone area has the identifier 0.0.0.0, Inter-area routing goes via the backbone.