Modulo II - Chapter 2 - 5

Question 1

Which of the following are used to protect against a broken fiber strand causing incorrect behavior of the spanning tree protocol? (Select 2 answers)
A - Root Guard
B - BPDU Guard
C - UDLD
D - Loop Guard

Answer 1

C and D
UDLD and Loop Guard are both used to protect against incorrect spanning tree behavior when a port is not sending BPDUs bidirectionally. Loop Guard is used on root and alternate ports and ensures that the neighboring switch is sending BPDUs. UDLD is configured on a physical interface and ensures that bidirectional communication is functional.

Question 2

Which of the following best describe the values of spanning-tree vlan vlan-id priority? (Select 2 answers)
A - The valid priority range is 0 to 61440.
B - The valid priority range is 0 to 65536.
C - The value should be configured in increments of 1024.
D - The value should be configured in increments of 4096.

Answer 2

A and D
The Bridge Priority field in the BPDU is split so that the first 4 bits represent the bridge priority, in increments of 4096, and the remaining 12 bits are used to represent the VLAN identifier. The valid range of values for priority are 0 to 61440.

Because ultimately this priority has to be added to the vlan number (extended system-id) to make final STP priority. So if we take 65536, then the minimum net priority will be 65536 + 1 = 65537 which cannot be fit in a 2-byte space.

Question 3

Which of the following actions ensure that the correct switch is chosen as the root bridge? (Select 2 answers)
A - Configure the network with BPDU Guard to prevent unauthorized switches from becoming root.
B - Configure the system priority to 0 on the primary root bridge.
C - Configure the system priority to 4096 on the primary root bridge.
D - Configure the network with BPDU Filter to prevent unauthorized switches from becoming root.

Answer 3

A and B
Incorreto
Configuring the root bridge with system priority 0 provides the best chance of its becoming the root. The BPDU Guard feature can be used to ensure that another switch cannot assume the role of root. To avoid potential loops, BPDU Filter should be used with caution. BPDU Filter filters BPDUs and should be used only in specific situations.

Question 4

Which of the following are valid 802.1D port types? (Select 2 answers)
A - Alternate port
B - Backup port
C - Root port
D - Forwarding port
E - Designated port

Answer 4

C and E
The valid IEEE 802.1D port types are root port and designated port. Alternate port and backup port are port roles defined in 802.1W RSTP.

Question 5

Which statements are true regarding the STP portfast feature? (Select 2 answers)
A - It is not possible to configure portfast on a trunk port.
B - Portfast disables STP on the port.
C - Portfast allows the port to bypass the listening and learning steps and move directly to forwarding.
D - TCNs are not generated when an edge port changes state.

Answer 5

C and D
The STP portfast feature is typically enabled on edge ports that are connected directly to end hosts, where the possibility of forming a loop does not exist. It can be combined with BPDU Guard to put a port in the ErrDisabled state if a switch is connected to the port. Edge ports do not generate TCNs to provide a more stable spanning tree environment. A TCN based on an edge port change would not bring any benefit since the overall spanning tree topology should not change. It is possible to enable portfast on a trunk port (for instance, connected to a firewall or server that needs to receive 802.1Q tags) and also meets the requirement as an edge port.

Question 6

Which of the following statements is true regarding topology change propagation for 802.1W RSTP? (Select 2 answers)
A - Edge ports that transition to the forwarding state cause topology changes.
B - The MAC address table entries associated with all ports are cleared, except for the port that received the BPDU with the TC flag set.
C - The TC while timer value is equal to twice the hello timer.
D - The root bridge in RSTP uses the TCN BPDU to propagate the change.

Answer 6

B and C
A switch that detects a topology change on a non-edge port sends BPDUs with the TC flag set for the duration of the TC while timer (hello timer * 2). MAC addresses associated with these ports are flushed. This occurs on all designated ports (DPs) and the root port (RP). Neighboring switches that receive the BPDU with the TC flag set clear the MAC table entries associated with all ports except the port that received the BPDU. The TC while timer is started on all DPs and the RP, and BPDUs are sent during this time with the TC flag set. This process continues until the change is propagated across the network. Unlike 802.1D, RSTP does not rely on the root bridge to send a specific TCN BPDU.

Question 7

What would be the value of the bridge priority in the BPDU generated in VLAN 10 when the default spanning tree bridge priority is used?
A - 32778
B - 32768
C - 4096
D - 24576

Answer 7

A
extended system-id borrows bits from the Bridge Priority field in the original 802.1D BPDU. This provides 4 bits to represent bridge priority, in increments of 4096, and 12 bits to represent the VLAN identifier. These values are added together to determine the priority. The default priority is 32768, so (32768 + 10) for VLAN 10 would result in the priority value 32778.

Question 8

The root bridge in an 802.1D STP network has which of the following characteristics? (Select 2 answers)
A - All of the ports of the root bridge are root ports.
B - If all switches in the network are using the default priority, the root bridge will have the highest MAC address among all the switches.
C - If all switches in the network are using non-default priority values, the root bridge will have the lowest system priority among all the switches.
D - All the ports of the root bridge are designated ports.

Answer 8

C and D
The root bridge has each port in the forwarding state, and all the ports are designated ports. The root bridge is determined by the switch with the lowest system priority value (the default is 32768). If all switches have the same system priority, the switch with the lowest MAC address becomes the root.

Question 9

Which would be the preferred way of implementing a loop-free switch network where there are 1500 VLANs defined and there is a need to load share the traffic through two major aggregation points based on VLAN identifier? (Select 2 answers)
A - 802.1D
B - 802.1s
C - 802.1W
D - 802.1AE

Answer 9

B and C
PRECISA VALIDAR A RESPOSTA

Question 10

Which of the following best describe the BPDUs used in 802.1D? (Select 2 answers)
A - There are two types of BPDUs.
B - 802.1D and 802.1W use the same BPDU format.
C - BPDUs are sent at each 2-second hello interval, even if no BPDU is received on the root port.
D - Configuration BPDUs identify the root bridge.

Answer 10

A and D
802.1D defines two BPDU types, the configuration BPDU and the topology change notification (TCN) BPDU. Several important fields are included in configuration BPDUs, including the root bridge identifier, timers, and cost to the root. 802.1D bridges only forward BPDUs from the root port, where 802.1W bridges generate a BPDU at each hello interval.

Question 11

Which of the following applies when connecting an MST region to a switch domain running 802.1W when the root bridge exists in the 802.1W domain? (Select 2 answers)
A - The MST region boundary receives superior BPDUs from the 802.1W switch and places the ports into a root inconsistent blocking state.
B - The spanning tree Loop Guard feature can be used to allow the boundary ports to transition to forwarding state.
C - The 802.1W domain automatically detects the presence of the MST region, based on the received BPDUs.
D - The MST region boundary switch automatically detects the presence of the 802.1W domain based on received BPDUs.

Answer 11

A and D
Upon receiving a superior BPDU from the 802.1W domain, the MST region boundary port moves to blocking, root inconsistent state. The PVST simulation mechanism works only when the root bridge is located in the MST region.

Question 12

Which of the following statements describe how the 802.1D root path cost is calculated? (Select 2 answers)
A - The default port cost of a 1 Gbps interface is 4.
B - The root port is chosen based on the highest path cost to the root bridge.
C - The root path cost is a cumulative value of the path cost received in a BPDU plus the local port cost.
D - The root bridge has a root path cost of 4096.

Answer 12

A and C
The root path cost is determined based on the cumulative port cost to reach the root bridge. Using the default short mode STP cost calculation, a 1 Gbps interface will use a cost of 4. The port cost is used in determining the path to the root bridge. The lowest path cost will be chosen.

Question 13

Which of the following best describe the VLAN-to-instance mapping in 802.1s? (Select 2 answers)
A - The configuration BPDUs generated by the root bridge contain the VLAN-to-instance mapping for the entire region.
B - All VLANs are mapped to instance 0 by default.
C - The VLAN-to-instance mapping must be manually configured on each switch in the region.
D - When interconnecting multiple MST regions, the VLAN-to-instance mapping must be unique to each region.

Answer 13

B and C
When MST is initially configured, all VLANs are mapped to instance 0. In addition, instance 0 is present on all links in the region even if a VLAN is pruned from that interface or switch. For each switch in the network, the VLAN-to-instance mapping must be configured to the same values.

Question 14

Switch 1 is capable of running the RSTP protocol. Switch 2 is not configured for RSTP and is connected to Switch 1 on port Gig1/0/1. What is the expected behavior for Gig1/0/1 on Switch 1?
A - Port Gig1/0/1 on Switch 1 will remain in the discarding state.
B - Switch 1 will receive Type 2 BPDUs on Gig1/0/1 from Switch 2.
C - Switch 1 will not receive a proposal agreement and will default to 802.1D behavior.
D - Port Gig1/0/1 on Switch 1 will transition immediately to the forwarding state.

Answer 14

C
A switch operating with RSTP 802.1W has the ability to detect ports that connect to legacy 802.1D switches. When this occurs, the switch reverts to 802.1D STP behavior. The ability to detect RSTP-capable or 802.1D ports relies on the exchange of BPDUs with the proposal and agreement flags set.

Question 15

Which of the following best describe the 802.1s internal spanning tree (IST) ? (Select 2 answers)
A - Information about all instances is carried in the BPDUs for the IST.
B - Each instance generates its own BPDUs.
C - IST uses instance 0 by default.
D - IST uses instance 15 by default.

Answer 15

A and C
IST is always instance 0 and runs on all interfaces in the MST region. Information about other instances is contained in the IST BPDUs, including the revision, which must match in all switch configurations.

Question 16

Which of the following statements are true regarding the convergence time of an 802.1D spanning tree network? (Select 2 answers)
A - Portfast should be used on access ports that are connected to end hosts to reduce the convergence time.
B - Convergence is typically around 50 seconds in duration.
C - Convergence is typically around 30 seconds in duration.
D - Portfast should be used on trunk ports connected to other switches to reduce the convergence time.

Answer 16

A and B
Convergence can take up to ((2 * Forward Delay) + Max_Age), or (( 2 * 15) + 20) = 50 seconds, based on default timer values. The portfast feature can speed up convergence by allowing ports to transition to forwarding immediately and bypassing the listening and learning states.

Question 17

Which of the following statements about the MST PVST simulation mechanism are true? (Select 2 answers)
A - The IST instance BPDUs are sent out for each VLAN into the RSTP domain by the MST boundary port.
B - The MST region boundary receives BPDUs for each VLAN from the RSTP switch and discards any BPDU received for a blocked VLAN. The remaining BPDUs are mapped 1:1 in the appropriate MSTIs.
C - The MST region boundary receives BPDUs for each VLAN from the RSTP switch and maps the BPDUs from VLAN 1 into the IST instance.
D - The IST instance BPDUs are sent out for VLAN 1 into the RSTP domain by the MST boundary port. These BPDUs are mapped from the IST instance.

Answer 17

A and C
When an MST boundary port receives PVST BPDUs, it triggers a simulation mechanism. The root bridge should exist in the MST region. For each PVST, the IST BPDUs are copied and sent to the neighboring switch. For BPDUs received by the boundary port, only the VLAN 1 BPDUs from the PVST domain are mapped to the IST. The user-defined MSTIs do not interact with any switch outside the MST region.

Question 18

Select the answers that best describe the loop guard feature. (Select 2 answers)
A - When BPDUs are not received, the port moves to the blocking status (loop inconsistent) until BPDUs are received again.
B - When BPDUs are not received, the port moves to the listening status (loop inconsistent) until BPDUs are received again.
C - Loop guard should always be configured on interfaces where portfast is enabled.
D - Loop guard is configured on root ports.

Answer 18

A and D
The STP loop guard feature is used to ensure that a root port or an alternate port is always receiving BPDUs from the neighboring switch. This provides protection against a link failing to send BPDUs bidirectionally, allowing a port to assume that no other bridge is connected. If BPDUs stop being received, the port moves to blocked status until BPDUs resume, and it shows a loop inconsistent state.

Question 19

Which of the following statements regarding LACP is true?
A - LACP is a Cisco-proprietary protocol.
B - LACP forms a dynamic EtherChannel if both ends are configured in the Passive mode.
C - LACP supports configuring a maximum number of member links that can exist in an EtherChannel.
D - LACP uses the multicast destination MAC address 0100:0CCC:CCCC.

Answer 19

C

Question 20

Select the statements that correctly describe UDLD configuration. (Select 2 answers)
A - UDLD must be enabled on both the local and remote switches’ ports to become functional.
B - UDLD should be enabled on a port-channel interface to protect against spanning tree failure.
C - Two modes exist: Aggressive and Normal.
D - Two modes exist: Aggressive and Passive.

Answer 20

A and C
UDLD can be configured in either Aggressive mode or Normal mode. Aggressive mode results in the port being in the ErrDisabled state upon failure. In order to become functional, UDLD must be enabled on both switches connected by a point-to-point Ethernet link. On a port-channel interface, UDLD is enabled on the member link interfaces.

Question 21

What are some of the benefits of configuring 802.3AD between two switches? (Select 2 answers)
A - 802.3AD allows multiple 802.1Q tags on a single Ethernet frame, known as Q-in-Q.
B - 802.3AD can improve the stability of the spanning tree topology.
C - 802.3AD provides a cost-effective way to increase bandwidth by logically combining multiple member links.
D - 802.3AD provides a way of load-balancing traffic on a single physical interface by using different wavelengths of light with a single fiber pair.

Answer 21

B and C
802.3AD provides the EtherChannel specification, also commonly referred to as link bundling or port channel. EtherChannel provides multiple benefits, including allowing multiple physical interfaces to be combined into a single logical bundle. This can not only increase bandwidth, but the EtherChannel is a single interface in the spanning tree topology. Thus, a single member link can go down without requiring a spanning tree topology change. Another benefit is that frames are load balanced across the member links, based on a configurable EtherChannel load balancing hash mechanism.

Question 22

Which statement about STP path cost calculation is true? (Select 2 answers)
A - The port priority and port number used in the path cost calculation are controlled by the upstream switch.
B - If all other parameters are equal, the highest port number is preferred.
C - If all other parameters are equal, the lowest port number is preferred.
D - The port number used in the path cost calculation comes from the local switch.

Answer 22

A and C
The STP path cost is calculated based on the port priority and the port number of the neighboring switch (in other words what is received from a BPDU). The final factor to determine the best port to reach the root is made based on the port number, where the lowest port is preferred.

Question 23

Which of the following statements best describe the Root Guard feature? (Select 2 answers)
A - Root Guard ensures that non-designated ports transition to forwarding if BPDUs are not received.
B - Root Guard moves a port to the ErrDisabled state if any STP BPDU is received.
C - Root Guard moves a port to the ErrDisabled state if a superior BPDU is received.
D - Root Guard can be configured on designated ports.

Answer 23

C and D
Root Guard can be configured on designated ports and does not allow the port to become non-designated. If a superior BPDU is received on the port, it is placed in the ErrDisabled state.

Question 24

When configuring spanning tree priority to assign the role of root bridge and secondary root bridge, which of the following should be considered? (Select 2 answers)
A - It is preferred to have the root bridge in the access switching layer so it does not get overwhelmed with traffic and miss BPDU processing.
B - The desired root bridge should have the highest system priority of all switches.
C - The secondary root bridge should be placed to avoid major changes in the STP topology upon failure of the primary.
D - The desired root bridge should have the lowest system priority of all switches.

Answer 24

C and D
The root bridge is the most important switch in the topology. It should ideally be placed in the core layer. The secondary root bridge should be placed to avoid any major changes in the spanning tree topology if the primary fails. The system priority of the root bridge should be lower than that of any switch in the topology, and the secondary root bridge should have the second lowest priority. All the remaining switches should be configured with a system priority higher than the default 32768.

Question 25

Which of the following describes the behavior of an interface where spanning-tree bpdufilter enable is configured?
A - The port continues to send BPDUs but ignores any received BPDUs.
B - The port does not send any BPDUs but processes any received BPDUs and moves the port to the ErrDisabled state.
C - The port does not send any BPDUs and ignores any received BPDUs.
D - BPDU filter should be configured on all non-designated ports to improve convergence of the spanning tree topology.

Answer 25

C
When BPDU filter is configured on an interface, the port does not send any BPDUs and ignores any received BPDUs. If the global configuration command spanning-tree portfast bpdufilter default is used, the port sends a series of BPDUs to see if any are received back. If it is the more preferred switch, it does not process any received BPDUs but still transmits BPDUs. If it is not the preferred switch, it processes received BPDUs and ceases to send BPDUs on the port. BPDU filter should be used with caution as it may allow forwarding loops to occur.