Which step in the link-state routing process is described by a router flooding link-state and cost information about each directly connected link?

Exchanging link-state advertisements is the step in the link-state routing process that is described by a router flooding link-state and cost information about each directly connected link. Routers use Link-State Advertisements (LSAs) to share information about their own links and the state of those links with all other routers in the same area. This allows all routers to have an identical database of the network topology, which can then be used to build a complete and synchronized view of the network.

• Electing the designated router is a process in OSPF used on multi-access networks like Ethernet. A designated router (DR) is elected among all routers on a particular segment to reduce the amount of LSA flooding. This is not the step where link-state and cost information is flooded; it’s a prelude to more efficient flooding.
• Injecting the default route refers to the process of a router introducing a default route (0.0.0.0/0) into the routing process. This is typically done to provide a gateway of last resort in a routing domain, not for flooding link-state information.
• Building the topology table is a term more associated with Enhanced Interior Gateway Routing Protocol (EIGRP), which is a distance-vector routing protocol, not a link-state protocol. In link-state protocols like OSPF or IS-IS, the equivalent process would be building the link-state database from received LSAs.