CompTIA Network+ N10-008 Exam Question & Answer: Switch not forwarding frames on trunked port

Question

A technician monitors a switch interface and notices it is not forwarding frames on a trunked port. However, the cable and interfaces are in working order.

Which of the following is MOST likely the cause of the issue?

A. STP policy
B. Flow control
C. 802.1Q configuration
D. Frame size

Answer

C. 802.1Q configuration

Explanation 1

The correct answer is C. 802.1Q configuration. The issue is most likely caused by the 802.1Q configuration on the switch port. 802.1Q is a protocol used for VLAN tagging on Ethernet networks. When a port is configured as a trunk port, it should be able to forward frames with VLAN tags. If the switch interface is not forwarding frames on a trunked port, it could be due to an incorrect 802.1Q configuration on the switch port.

STP policy is used to prevent loops in a network and would not cause the issue described in the question. Flow control is used to manage the flow of data between devices and would not cause the issue described in the question. Frame size would not cause the issue described in the question as it does not affect the forwarding of frames on a trunked port.

Explanation 2

The correct answer is C. 802.1Q configuration.

802.1Q is a standard that allows multiple VLANs to be carried over a single trunked link. When a switch interface is configured as a trunk, it must be configured with the appropriate 802.1Q tags for the VLANs that will be carried over the link. If the 802.1Q configuration is incorrect, the switch will not forward frames on the trunked port.

The other options are less likely to be the cause of the issue. STP policy is used to prevent loops in a network, and it would not prevent a switch from forwarding frames on a trunked port. Flow control is used to prevent a switch from being overwhelmed with traffic, and it would not prevent a switch from forwarding frames on a trunked port. Frame size is not a factor that would prevent a switch from forwarding frames on a trunked port.

Here are some additional details about 802.1Q tagging:

802.1Q tags are 32-bit fields that are added to the header of Ethernet frames.
The 802.1Q tag identifies the VLAN that the frame belongs to.
Switches use 802.1Q tags to separate frames from different VLANs.
When a switch interface is configured as a trunk, it must be configured with the appropriate 802.1Q tags for the VLANs that will be carried over the link.

Explanation 3

The issue of a switch interface not forwarding frames on a trunked port can be caused by several factors. However, the most likely cause of the issue is 802.1Q configuration.

The 802.1Q protocol is used to tag frames with VLAN information as they traverse a network. If the switch interface is not configured to accept tagged frames, it will not forward them on the trunked port.

Explanation 4

The correct answer is C. 802.1Q configuration.

802.1Q is a standard that allows multiple VLANs to be carried over a single trunked link. When a switch interface is configured as a trunk, it must be configured with the appropriate 802.1Q tags for the VLANs that will be carried over the link. If the interface is not configured with the correct tags, it will not forward frames for those VLANs.

Here are some other possible causes of the issue:

STP policy: If the switch is configured with a STP policy that prevents frames from being forwarded between certain VLANs, the interface may not be forwarding frames for those VLANs.
Flow control: If the switch is configured with flow control, it may be preventing frames from being forwarded to the interface if the interface is receiving too much traffic.
Frame size: If the frames that are being received on the interface are larger than the maximum frame size that the interface can handle, the interface may not be forwarding the frames.

To troubleshoot the issue, you can use the following steps:

Verify that the cable and interfaces are in working order.
Verify that the switch is configured with the correct 802.1Q tags for the VLANs that will be carried over the link.
Verify that the switch is not configured with a STP policy that prevents frames from being forwarded between certain VLANs.
Verify that the switch is not configured with flow control that is preventing frames from being forwarded to the interface.
Verify that the frames that are being received on the interface are not larger than the maximum frame size that the interface can handle.

If you have verified all of these things and the interface is still not forwarding frames, you may need to contact the switch manufacturer for further assistance.

Explanation 5

Based on the information provided, the most likely cause of the issue is C. 802.1Q configuration.

Explanation:

Trunked ports are used to carry traffic for multiple VLANs (Virtual Local Area Networks) over a single physical link. The fact that frames are not being forwarded on a trunked port indicates a problem with the configuration.

Let’s examine the other options and explain why they are less likely to be the cause of the issue:

A. STP policy:
Spanning Tree Protocol (STP) is a network protocol that prevents loops in a Layer 2 network by blocking redundant paths. While STP can affect the forwarding of frames, it is less likely to be the cause of the issue in this scenario. If the STP policy was the problem, it would affect all frames on the switch, not just the frames on the trunked port. Additionally, STP issues typically result in intermittent connectivity or complete loss of connectivity rather than preventing frames from being forwarded.

B. Flow control:
Flow control is a mechanism used to manage the flow of data between network devices to prevent packet loss or congestion. However, flow control is generally not related to the forwarding of frames on a trunked port. Flow control issues are more likely to impact the speed of data transfer or cause congestion, but they would not specifically prevent frames from being forwarded on a trunked port.

D. Frame size:
Frame size refers to the length of the data portion of a network frame. While frame size can impact network performance and efficiency, it is unlikely to be the cause of the issue described. If there were issues with frame size, it would affect all frames, not just those on the trunked port. Moreover, frame size problems are typically related to issues like fragmentation or maximum transmission unit (MTU) mismatches, which would result in partial or complete loss of connectivity, not just the failure to forward frames on a specific port.

C. 802.1Q configuration:
The 802.1Q is a standard for VLAN tagging in Ethernet networks. VLANs allow for logical separation of network traffic, and 802.1Q tagging is used to identify which VLAN a particular frame belongs to. If there is a misconfiguration or a mismatch in the 802.1Q configuration on the switch and the connected device, it can prevent the switch from properly forwarding frames on the trunked port. This misconfiguration could result in the frames not being recognized as part of the correct VLAN, causing them to be dropped or ignored.

To resolve the issue, the technician should verify the 802.1Q configuration on both the switch and the connected device, ensuring that they are using the same VLAN configuration and that the trunked port is properly configured to allow the desired VLAN traffic. Additionally, checking for any errors or warnings in the switch logs related to VLANs or the trunked port can provide further insights into the cause of the issue. By resolving the 802.1Q configuration problem and ensuring consistency between the switch and the connected device, the frames should be forwarded correctly on the trunked port.

Explanation 6

The correct answer is C. 802.1Q configuration.

STP policy, flow control, and frame size are all possible causes of a switch interface not forwarding frames, but they are less likely than an incorrect 802.1Q configuration.

Here are some additional details about the 802.1Q standard:

802.1Q is a layer-2 standard that adds a 4-byte header to Ethernet frames. This header contains information about the VLAN that the frame belongs to.
802.1Q is used to create trunked links between switches. A trunked link is a link that carries multiple VLANs.
To configure a switch interface as a trunk, you need to use the `switchport trunk allowed vlan` command. This command specifies the VLANs that will be carried over the trunked link.

If you are having problems with a switch interface not forwarding frames, you should check the 802.1Q configuration for that interface. Make sure that the interface is configured as a trunk and that the correct VLANs are allowed on the trunk.

Explanation 7

The answer to the question is C. 802.1Q configuration. This is because 802.1Q is a standard protocol for creating VLANs (virtual LANs) on a trunked port. A trunked port is a port that can carry traffic from multiple VLANs using tags to identify the VLAN membership of each frame. If the 802.1Q configuration is incorrect on either end of the trunked port, the frames will not be forwarded correctly and will be dropped by the switch.

A detailed explanation of why the other options are wrong is as follows:

A. STP policy: STP (spanning tree protocol) is a protocol that prevents loops in a network by blocking some ports on switches. STP policy does not affect the forwarding of frames on a trunked port, unless the port is blocked by STP due to a loop.

B. Flow control: Flow control is a mechanism that regulates the rate of data transmission between two devices to avoid congestion and data loss. Flow control does not affect the forwarding of frames on a trunked port, unless the port is congested and cannot send or receive more frames.

D. Frame size: Frame size refers to the amount of data that can be carried in a single frame. Frame size does not affect the forwarding of frames on a trunked port, unless the frame size exceeds the MTU (maximum transmission unit) of the port and causes fragmentation or discarding of frames.

Explanation 8

A switch is a network device that connects multiple devices on a local area network (LAN) and forwards frames based on their destination MAC addresses. A trunked port is a switch port that can carry traffic from multiple VLANs (virtual LANs) using a tagging protocol such as 802.1Q. VLANs are logical segments of a LAN that allow devices to communicate as if they were on separate physical networks.

A technician monitors a switch interface and notices it is not forwarding frames on a trunked port. However, the cable and interfaces are in working order. This means that there is a problem with the configuration or operation of the switch or the trunked port that prevents it from sending or receiving frames.

Based on the options given, I think the most likely cause of the issue is C. 802.1Q configuration. This is because 802.1Q is a protocol that adds a tag to each frame that identifies its VLAN membership. The tag consists of a 4-byte header that contains a 12-bit VLAN ID and other information. If the 802.1Q configuration on the switch or the trunked port is incorrect or mismatched with the other devices on the network, the switch may not be able to recognize or forward frames with different VLAN tags.

The other statements are less likely because:

A. STP policy: This statement is less likely because STP (Spanning Tree Protocol) is a protocol that prevents loops in a network by blocking redundant links between switches. STP policy determines which links are blocked and which are active based on factors such as port priority and path cost. If STP policy was the cause of the issue, the technician would notice that the trunked port was in a blocking state, not forwarding any frames at all.

B. Flow control: This statement is less likely because flow control is a mechanism that regulates the rate of data transmission between two devices to avoid congestion or buffer overflow. Flow control can be implemented at different layers of the OSI model using different methods such as windowing or backpressure. If flow control was the cause of the issue, the technician would notice that the trunked port was experiencing delays or drops in frame transmission due to congestion or buffer overflow.

D. Frame size: This statement is less likely because frame size is the amount of data contained in a frame excluding the header and trailer. Frame size can vary depending on factors such as MTU (Maximum Transmission Unit) or jumbo frames. If frame size was the cause of the issue, the technician would notice that some frames were too large or too small to be forwarded by the switch or received by the destination device.

Reference

CompTIA Network+ N10-008 certification exam practice question and answer (Q&A) dump with detail explanation and reference available free, helpful to pass the CompTIA Network+ N10-008 exam and earn CompTIA Network+ N10-008 certification.

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