5.4.2 Module Quiz – STP Answers
1. Which port state will switch ports immediately transition to when configured for PortFast?
- forwarding
- blocking
- listening
- learning
Explanation: PortFast allows a switch port to bypass the listening and learning states and transition immediately to the forwarding state.
2. After the election of the root bridge has been completed, how will switches find the best paths to the root bridge?
- Each switch will analyze the port states of all neighbors and use the designated ports to forward traffic to the root.
- Each switch will analyze the sum of all port costs to reach the root and use the path with the lowest cost.
- Each switch will analyze the sum of the hops to reach the root and use the path with the fewest hops.
- Each switch will analyze the BID of all neighbors to reach the root and use the path through the lowest BID neighbors.
Explanation: After the election of a root bridge has occurred, each switch will have to determine the best path to the root bridge from its location. The path is determined by summing the individual port costs along the path from each switch port to the root bridge.
3. Which is the default STP operation mode on Cisco Catalyst switches?
- MST
- PVST+
- Rapid PVST+
- MSTP
- 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.
4. What value determines the root bridge when all switches connected by trunk links have default STP configurations?
- VLAN ID
- MAC address
- extended system ID
- bridge priority
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 determine which switch is the root for that VLAN.
5. During the implementation of Spanning Tree Protocol, all switches are rebooted by the network administrator. What is the first step of the spanning-tree election process?
- All the switches send out BPDUs advertising themselves as the root bridge.
- Each switch determines the best path to forward traffic.
- Each switch determines what port to block to prevent a loop from occurring.
- Each switch with a lower root ID than its neighbor will not send BPDUs.
Explanation: After a Cisco switch boots, it will send out BPDUs containing its individual BID and the root ID for the network. By default, the initial root ID at bootup will be the ID of that individual switch. After a root bridge is elected, port states and paths are chosen.
6. 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
- PortFast
- edge port
- extended system ID
- 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 time-consuming 802.1D listening and learning port states. PVST+ is the default spanning-tree configuration for a Cisco Catalyst switch. The bridge ID (BID) is used to determine the root bridge on a network and includes the bridge priority, the extended system ID, and the MAC address.
7. Which three port states are used by Rapid PVST+? (Choose three.)
- listening
- blocking
- trunking
- learning
- forwarding
- discarding
Explanation: The Rapid PVST+ port states are discarding, learning, and forwarding.
8. When PVST is running over a switched network, which port state can participate in BPDU frame forwarding based on BPDUs received, but does not forward data frames?
- disabled
- forwarding
- listening
- blocking
Explanation: Ports in the blocking state are nondesignated ports and do not participate in frame forwarding. Ports in the listening state can participate in BPDU frame forwarding according to received BPDU frames, but do not forward data frames. Ports in the forwarding state forward data frames and send and receive BPDU frames. Ports in the disabled state are administratively disabled.
9. Which STP port role is adopted by a switch port if there is no other port with a lower cost to the root bridge?
- designated port
- alternate
- disabled port
- root port
Explanation: The root port is the port with the lowest cost to reach the root bridge.
10. Which two statements describe a switch port that is configured with PortFast? (Choose two.)
- The switch port immediately transitions from the listening to the forwarding state.
- The switch port immediately transitions from blocking to the forwarding state.
- The switch port sends DHCP requests before transitioning to the forwarding state.
- The switch port immediately processes any BPDUs before transitioning to the forwarding state.
- The switch port should never receive BPDUs.
Explanation: A port that is configured with PortFast will immediately transition from blocking to the forwarding state. PortFast should only be configured on switch ports that support end devices, so no BPDUs should ever be received through a port that is configured with PortFast. Configuring a port with PortFast supports DHCP because PortFast will speed up the transition from blocking to forwarding. Without PortFast, an end device may begin to issue DHCP requests before the port has transitioned to the forwarding state.
11. What is one way to correct a spanning tree failure?
- Replace the cables on failed STP links.
- Manually remove redundant links in the switched network.
- Insert redundant links to replace the failed STP links.
- Replace all instances of STP with RSTP.
Explanation: An action that can be taken when there is a spanning tree failure in a Layer 2 network is to remove all redundant links in the failed segment of the network. This will eliminate the loops in the topology allowing for a normalization of the traffic and CPU loads. The next step would be to investigate the failure of STP on the redundant links and fix these issues prior to restoring these links.
12. What additional information is contained in the 12-bit extended system ID of a BPDU?
- port ID
- MAC address
- IP address
- VLAN ID
Explanation: The BPDU has three fields; the bridge priority, the extended system ID, and the MAC address. The extended system ID contains 12 bits that identify the VLAN ID.
13. An administrator is troubleshooting a switch and wants to verify if it is a root bridge. What command can be used to do this?
- show spanning-tree
- show running-config
- show vlan
- show startup-config
Explanation: Of all the commands that are listed, only the correct option, show spanning-tree, displays STP root bridge information.
14. What is an accurate description of redundancy?
- designing a network to use multiple virtual devices to ensure that all traffic uses the best path through the internetwork
- configuring a router with a complete MAC address database to ensure that all frames can be forwarded to the correct destination
- designing a network to use multiple paths between switches to ensure there is no single point of failure
- configuring a switch with proper security to ensure that all traffic forwarded through an interface is filtered
Explanation: Redundancy attempts to remove any single point of failure in a network by using multiple physically cabled paths between switches in the network.
15. 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.
16. What is an advantage of PVST+?
- PVST+ optimizes performance on the network through automatic selection of the root bridge.
- PVST+ optimizes performance on the network through load sharing using multiple root bridges.
- 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 it increases the CPU cycles for all the switches in the network.
17. 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.
18. What two features does Spanning Tree Protocol (STP) provide to ensure proper network operations? (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.
19. Which PVST+ 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 will put an edge port in an error-disabled state if a BPDU is received by the port. This will prevent a Layer 2 loop occurring.
20. What is an advantage of using STP in a LAN?
- It combines multiple switch trunk links into a logical port channel link to increase bandwidth.
- It decreases the size of the failure domain.
- It provides firewall services to protect the LAN.
- It temporarily disables redundant paths to stop Layer 2 loops.
Explanation: STP allows redundant physical connections between Layer 2 devices without creating Layer 2 loops by disabling ports that could create a loop.
21. Which two statements regarding a PortFast enabled switch port are true? (Choose two.)
- The port immediately transitions from blocking to forwarding state.
- The port immediately transitions from listening to forwarding state.
- The port immediately processes any BPDUs before transitioning to the forwarding state.
- The port sends DHCP requests before transitioning to the forwarding state.
- The port should never receive BPDUs.
Explanation: PortFast-enabled ports immediately transition from blocking to forwarding state. PortFast should be enabled only on access ports connecting end devices. No BPDUs should ever be received through a port that is configured with PortFast.
An administrator is troubleshooting a switch and wants to verify if it is a root bridge. What command can be used to do this?
Answer: show spanning-tree
Which STP port role is adopted by a switch port if there is no other port with a lower cost to the root bridge?
Answer: root port