CCNA 3 v6 Chapter 3: Check Your Understanding Questions Answers

CCNA 3 v6.0 (Scaling Networks v6) Chapter 3: STP: Check Your Understanding Questions Answers

1. What could be the effect of duplicate unicast frames arriving at a destination device due to multiple active alternative physical paths?

  • Application protocols malfunction.
  • Frame collisions increase.
  • The number of broadcast domains increases.
  • The number of collision domains increases.

Explanation: When multiple alternative physical paths exist, duplicate unicast frames from a sending host are sent to the destination. Many application protocols expect to receive only one copy of each packet, especially TCP-based protocols that use sequence number and acknowledge number to track the sequence of packets. Multiple copies of the same frame could cause application protocols to make errors in processing the packets. Broadcast storms are caused by switches flooding broadcast frames endlessly. With microsegmentation, collisions are controlled within each switch port.

2. What additional information is contained in the 12-bit extended system ID of a BPDU?

  • IP address
  • MAC address
  • Port ID

Explanation: The BPDU has three fields: bridge priority, extended system ID, the MAC address. The extended system ID contains 12 bits that identify the VLAN ID.

3. Which three components are combined to form a bridge ID? (Choose three.)

  • Bridge priority
  • Cost
  • Extended system ID
  • IP address
  • MAC address
  • Port ID

Explanation: The three components that are combined to form a bridge ID are bridge priority, extended system ID, and MAC address.

4. Which STP port role is adopted by a switch port if there is no other port with a lower cost to the root bridge?

  • Alternate port
  • Designated port
  • Disabled port
  • Root port

Explanation: The root port is the port with the lowest cost to reach the root bridge.

5. Which is the default STP operation mode on Cisco Catalyst switches?

  • MST
  • MSTP
  • PVST+
  • Rapid PVST+
  • RSTP

Explanation: Cisco switches running IOS 15.0 or later run PVST+ by default. Cisco Catalyst switches support PVST+, Rapid PVST+, and MSTP. However, only one version can be active at any time.

6. What is an advantage of PVST+?

  • PVST+ optimizes performance on the network through autoselection of the root bridge.
  • PVST+ optimizes performance on the network through load sharing.
  • PVST+ reduces bandwidth consumption compared to traditional implementations of STP that use CST.
  • PVST+ requires fewer CPU cycles for all the switches in the network.

Explanation: PVST+ results in optimum load balancing. However, this is accomplished by manually configuring switches to be elected as root bridges for different VLANs on the network. The root bridges are not automatically selected. Furthermore, having spanning-tree instances for each VLAN actually consumes more bandwidth and increases the CPU cycles for all the switches in the network.

7. In which two port states does a switch learn MAC addresses and process BPDUs in a PVST network? (Choose two.)

  • Blocking
  • Disabled
  • Forwarding
  • Learning
  • Listening

Explanation: Switches learn MAC addresses at the learning and forwarding port states. They receive and process BPDUs at the blocking, listening, learning, and forwarding port states.

8. Which STP priority configuration would ensure that a switch would always be the root switch?

  • spanning-tree vlan 10 priority 0
  • spanning-tree vlan 10 priority 4096
  • spanning-tree vlan 10 priority 61440
  • spanning-tree vlan 10 root primary

Explanation: Although the spanning-tree vlan 10 root primary command ensures that a switch will have a bridge priority value lower than other bridges introduced to the network, the spanning-tree vlan 10 priority 0 command ensures that the bridge priority takes precedence over all other priorities.

9. To obtain an overview of the spanning-tree status of a switched network, a network engineer issues the show spanning-tree command on a switch. Which two items of information does this command display? (Choose two.)

  • The IP address of the management VLAN interface
  • The number of broadcasts received on each root port
  • The role of the ports in all VLANs
  • The root bridge BID
  • The status of native VLAN ports

Explanation: The show spanning-tree command displays the status of STP for all VLANs that are defined on a switch and other information, including the root bridge BID. It does not show the number of broadcast packets received on the ports. The IP address of the management VLAN interface is not related to STP and is displayed by the show running-configuration command.

10. Which two network design features require Spanning Tree Protocol (STP) to ensure correct network operation? (Choose two.)

  • Implementing VLANs to contain broadcasts
  • Link-state dynamic routing that provides redundant routes
  • Redundant links between Layer 2 switches
  • Removing single points of failure with multiple Layer 2 switches
  • Static default routes

Explanation: Spanning Tree Protocol (STP) is required to ensure correct network operation when designing a network with multiple interconnected Layer 2 switches or using redundant links to eliminate single points of failure between Layer 2 switches. Routing is a Layer 3 function and does not relate to STP. VLANs do reduce the number of broadcast domains but relate to Layer 3 subnets, not STP.

11. What value determines the root bridge when all switches connected by trunk links have default STP configurations?

  • Bridge priority
  • Extended system ID
  • MAC address

Explanation: When all switches are configured with the same default bridge priority, the MAC address becomes the deciding factor for the election of the root bridge. All links on the same VLAN will also have the same extended system ID, so this will not contribute to determining which switch is the root for that VLAN.

12. Which two concepts relate to a switch port that is intended to have only end devices attached and intended never to be used to connect to another switch? (Choose two.)

  • Bridge ID
  • Edge port
  • Extended system ID
  • PortFast
  • PVST+

Explanation: The RSTP edge port concept corresponds to the PVST+ PortFast feature. An edge port connects to an end station and assumes that the switch port does not connect to another switch. RSTP edge ports should immediately transition to the forwarding state, thereby skipping the timeconsuming 802.1D listening and learning port states.

13. Which Cisco switch feature ensures that configured switch edge ports do not cause Layer 2 loops if a port is mistakenly connected to another switch?

  • BPDU guard
  • Extended system ID
  • PortFast
  • PVST+

Explanation: If switch access ports are configured as edge ports using PortFast, BPDUs should never be received on those ports. Cisco switches support a feature called BPDU guard. When it is enabled, BPDU guard puts an edge port in an error-disabled state if a BPDU is received by the port. This prevents a Layer 2 loop from occurring.


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