CCNA 3 Chapter 3 v6 Exam Answers

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

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.

What port type is used to interconnect switches in a switch stack?

Switches configured to operate in a switch stack are connected together through StackWise ports.

Which port state will switch ports immediately transition to when configured for PortFast?

PortFast allows a switch port to bypass the listening and learning states and transition immediately to the forwarding state.

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

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.

Fill in the blank. Do not use abbreviations.
The spanning-tree __ global configuration command is used to enable Rapid

Switches configured to operate in a switch stack are connected together through StackWise ports.

Refer to the exhibit. Which switch will be elected the root bridge and which switch will place a port in blocking mode? (Choose two.)

The spanning-tree root bridge election process determines which switch becomes root bridge based first on the lowest priority number and then by lowest MAC address. Because all of the switches have the same priority value, SW3 becomes the root bridge based on lowest MAC address. To determine which ports are blocking or forwarding, first determine which ports become the root port on each switch. Then determine which port becomes the designated port for each link.

Which protocol provides up to 16 instances of RSTP, combines many VLANs with the same physical and logical topology into a common RSTP instance, and provides support for PortFast, BPDU guard, BPDU filter, root guard, and loop guard?

MST is the Cisco implementation of MSTP, an IEEE standard protocol that provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. STP and RSTP assume only one spanning-tree instance for the entire bridged network, regardless of the number of VLANs. PVST+ provides a separate 802.1D spanning-tree instance for each VLAN that is configured in the network.

Match the spanning-tree feature with the protocol type. (Not all options are used.)

What is the purpose of the Spanning Tree Protocol (STP)?

The Spanning-Tree Protocol (STP) creates one path through a switch network in order to prevent Layer 2 loops.

What is the outcome of a Layer 2 broadcast storm?

When the network is saturated with broadcast traffic that is looping between switches, new traffic is discarded by each switch because it is unable to be processed.

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Refer to the exhibit. Which trunk link will not forward any traffic after the root bridge election process is complete?

S4 has the lowest bridge ID, thus S4 is the root bridge. Because the path cost S1-S2-S4 is lower than the path cost S1-S3-S4, path S1-S2-S4 is the preferred path for S1 to reach S4. Thus, STP will set the S1 port Fa0/1 to a blocking state, and the trunk link Trunk2 will not forward any traffic.

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?

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. PVST+ is an implementation of the Spanning Tree Protocol. The extended system ID is a mechanism of including VLAN ID information in the bridge ID (BID) for each VLAN.

If no bridge priority is configured in PVST, which criteria is considered when electing the root bridge?

Only one switch can be the root bridge for a VLAN. The root bridge is the switch with the lowest BID. The BID is determined by priority and the MAC address. If no priority is configured then all switches use the default priority and the election of the root bridge will be based on the lowest MAC address.

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 will this command display? (Choose two.)

The show spanning-tree command will display 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.

Which RSTP ports are connected to end devices?

Refer to the exhibit. What is the role of the SW3 switch?

Switch SW3 is the STP root as can be seen in the show spanning-tree command output.

Which two types of spanning tree protocols can cause suboptimal traffic flows because they assume only one spanning-tree instance for the entire bridged network? (Choose two.)

STP and RSTP assume only one IEEE 802.1D spanning-tree instance for the entire bridged network irrespective of the number of VLANs, This can result in suboptimal traffic flow issues. PVST+ provides a separate spanning-tree instance for each VLAN configured. Rapid PVST+ provides a separate instance of 802.1w per VLAN, and MSTP maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance but allows for more than one instance for dissimilar traffic flows.

What is an advantage of PVST+?

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.

Open the PT Activity. Perform the tasks in the activity instructions and then answer the question.
Which switch is the root bridge?​

Click on each PC.
Use the Terminal application.
Issue the command show spanning-tree.
The switch that is the root bridge will be stated in the root bridge MAC address field.

Which three components are combined to form a bridge ID?

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

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What is a characteristic of a Layer 2 loop?

A Layer 2 loop occurs when broadcast frames are forwarded back to the sending switch via a backup link. Layer 2 frames do not have a Time-to-Live (TTL) attribute. Routers forward packets at Layer 3 (the network layer) not Layer 2.

Which spanning tree standard supports only one root bridge so that traffic from all VLANs flows over the same path?

MST is the Cisco implementation of MSTP, an IEEE standard protocol that provides up to 16 instances of RSTP. PVST+ provides a separate 802.1D spanning-tree instance for each VLAN that is configured in the network. 802.1D is the original STP standard defined by the IEEE and allows for only one root bridge for all VLANs. 802.1w, or RSTP, provides faster convergence but still uses only one STP instance for all VLANs.

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

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.

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

Although the spanning-tree vlan 10 root primary command will ensure 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 the bridge priority takes precedence over all other priorities.

A network administrator is preparing the implementation of Rapid PVST+ on a production network. How are the Rapid PVST+ link types determined on the switch interfaces?

When Rapid PVST+ is being implemented, link types are automatically determined but can be specified manually. Link types can be either point-to-point, shared, or edge.