Table of Contents
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
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.
Reference
- Troubleshoot Switch Port and Interface Problems – Cisco
- Solved: Trunk Port not forwarding VLAN traffic – Cisco Community
- Network+ (Plus) Certification | CompTIA IT Certifications
- Network+ (N10-008) Certification Study Guide | CompTIA IT Certifications
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