Cisco CCNA Spanning Tree Protocol (STP) Questions and Answers

Question 1

Three switches are connected in a full mesh topology. SwitchA has a priority of 32768 and MAC address 0000.AAAA.AAAA. SwitchB has a priority of 28672 and MAC address 0000.BBBB.BBBB. SwitchC has a priority of 32768 and MAC address 0000.CCCC.CCCC. Which switch will be elected as the Spanning Tree Protocol (STP) root bridge?

Accepted Answer

SwitchB

Answer

The STP root bridge election is based on the lowest Bridge ID (BID). The BID is composed of a 2-byte bridge priority and the 6-byte MAC address. The switch with the lowest priority becomes the root. In this case, SwitchB has a priority of 28672, which is lower than the priority of 32768 for both SwitchA and SwitchC. Therefore, SwitchB will be elected as the root bridge.

Question 2

A network administrator wants to prevent a user-facing access port from causing a Layer 2 loop if another switch is accidentally connected. The desired behavior is for the port to shut down immediately if it receives a BPDU. Which Cisco feature should be configured on this port?

Accepted Answer

BPDU Guard

Answer

BPDU Guard is a Cisco feature designed for access ports, especially those with PortFast enabled. If a port with BPDU Guard enabled receives any BPDU, it will immediately be placed into an err-disabled state, effectively shutting it down. This prevents unauthorized switches from influencing the STP topology.

Question 3

In the context of the original 802.1D Spanning Tree Protocol, which of the following best describes a Root Port?

Accepted Answer

The single port on a non-root switch that has the lowest path cost to the root bridge.

Answer

Every non-root switch in an STP topology must select one Root Port. The Root Port is the interface on that switch that has the lowest calculated path cost to reach the root bridge. It represents the best path from that switch toward the root.

Question 4

Rapid Spanning Tree Protocol (RSTP, 802.1w) improves convergence over the original 802.1D standard by modifying port states. Which three 802.1D port states are consolidated into the single 'Discarding' state in RSTP?

Accepted Answer

Disabled, Blocking, and Listening

Answer

RSTP streamlines the port state machine for faster convergence. The 802.1D states of Disabled, Blocking, and Listening are functionally combined into a single state in RSTP called Discarding. In this state, the port does not forward user frames or learn MAC addresses.

Question 5

A network topology consists of SwitchA (Root Bridge), SwitchB, and SwitchC. SwitchB connects to SwitchA via a 1 Gbps link. SwitchC connects to SwitchB via a 100 Mbps link. Assuming default STP costs are used, what is the root path cost for SwitchC?

Accepted Answer

23

Answer

The root path cost is the cumulative cost of all links to the root bridge. The default short path cost for a 1 Gbps link is 4, and for a 100 Mbps link is 19. SwitchB's cost to the root is 4. SwitchC's total cost is its own link cost to SwitchB plus SwitchB's cost to the root. Therefore, SwitchC's root path cost is 19 (SwitchC-to-SwitchB link) + 4 (SwitchB-to-SwitchA link) = 23.

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Cisco CCNA Spanning Tree Protocol (STP) Questions and Answers