Which two statements are true regarding BPDUs of spanning tree protocols? (Choose two.)

A.
Configuration BPDUs function as keepalives in STP and RSTP.

B.
BPDUs are sent by the designated bridge but not the backup bridge on shared segments.

C.
Configuration BPDUs are sent by both switches on point-to-point connections.

D.
Configuration BPDUs are not sent out interfaces configured as edge ports.

Explanation:
Bridge Protocol Data Units

The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network. The bridges have to determine the root bridge and compute the port roles (root, designated, or blocked) with only the information that they have. To ensure that each bridge has enough information, the bridges use special data frames called Bridge Protocol Data Units (BPDUs) to exchange information about bridge IDs and root path costs.

A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP multicast address 01:80:C2:00:00:00.

There are three types of BPDUs:

* Configuration BPDU (CBPDU), used for Spanning Tree computation
* Topology Change Notification (TCN) BPDU, used to announce changes in the network topology
* Topology Change Notification Acknowledgment (TCA)

BPDUs are exchanged regularly (every 2 seconds by default) and enable switches to keep track of network changes and to start and stop forwarding at ports as required.