Cisco CCNA mock exam questions sample test – Question 150

.Which protocol is used in redundant network topologies to avoid receiving multiple copies of the same frame?

A. 802.1q
B. Spanning Tree Protocol
C. Cisco Discovery Protocol
D. Routing Information Protocol

Correct Answer: B

Spanning Tree Protocol (STP) is used to remove switching loops in redundantly configured switched environments, and to create a single active Layer 2 path between any two
network segments. This eliminates the chance of multiple copies of the same unicast frame being sent in the LAN. It also prevents broadcast packets from creating a broadcast storm
when redundant connections exist between the switches. The benefits of STP include:
Prevention of broadcast storms
Prevention of multiple frame copies
Media Access Control (MAC) address database stability
Whenever a network segment can be handled by more than one switch, STP will elect one switch to take responsibility, and the other switches will be placed into a blocking state for
the ports connected to that segment. In this way, only one switch receives and forwards data for this segment, which removes the potential for multiple copies of the same frame being
generated. For STP to provide this functionality it must be running on all of the switches. Therefore, a properly implemented redundant topology STP is required in order to prevent
multiple copies of the same unicast frame from being transmitted.
802.1q is a frame tagging method for identifying Virtual LAN (VLAN) memberships over trunk links. Frame tagging ensures identification of individual VLAN frames over a trunk link
carrying frames for multiple VLANs. This frame tagging method is a standardized protocol that was developed by The Institute of Electrical and Electronics Engineers (IEEE). Cisco
has also developed a proprietary frame tagging method known as Inter-Switch Link (ISL). 802.1q does not mitigate loops or the reception of multiple copies of frames. The IEEE
specification for STP is 802.1d.
Cisco Discovery Protocol is a Cisco proprietary protocol used to collect hardware and protocol information for directly connected Cisco devices. CDP has nothing to do with redundant
network topologies.
Routing Information Protocol (RIP) is a distance vector routing protocol. It populates routing tables dynamically about the topology changes. However, RIP does not control the receipt
of multiple copies of frames in redundant network topologies.
LAN Switching Fundamentals
Configure, verify, and troubleshoot STP protocols
Cisco > Support > Configuring Spanning Tree-Protocol > How STP Works