What type of messages does BSR use to communicate between routers?

The correct answer pertains to the nature of how the Bootstrap Router (BSR) communicates. BSR utilizes messages that are directed to a specific endpoint or address within the network, rather than relying on general methods of communication like broadcasts or multicasts.

When BSR operates in the Protocol-Independent Multicast (PIM) framework, it is responsible for controlling the distribution of multicast group membership information. The use of unicast messages allows BSR to communicate directly with routers that need the information regarding group membership and to ensure a more efficient propagation of this data without flooding the network.

In contrast, broadcast messages would send data to all devices on the network segment, which can lead to unnecessary processing by all routers and potential network congestion. Multicast messages, while they serve to deliver data to multiple specific receivers efficiently, are not the primary method employed for BSR communication. Hop-by-hop messages typically describe a methodology for routing where each router forwards the packet based on its routing table, which in the context of BSR is not how the configuration is transmitted.

Thus, the method chosen by BSR effectively maintains a cleaner and more efficient signaling process between routers regarding multicast group information.