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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is troubleshooting inter-switch communication between Switch-Alpha and Switch-Beta. Users on VLAN 20 attached to Switch-Alpha report complete loss of connectivity to servers on VLAN 20 attached to Switch-Beta. Additionally, syslog messages indicate that untagged management frames transmitted from Switch-Alpha on VLAN 10 are being received on VLAN 30 on Switch-Beta across trunk interface GigabitEthernet 0/1.

An inspection of the interface configuration reveals:
- Switch-Alpha (GigabitEthernet 0/1): Mode: Trunk, Native VLAN: 10, Allowed VLANs: 10, 20, 30
- Switch-Beta (GigabitEthernet 0/1): Mode: Trunk, Native VLAN: 30, Allowed VLANs: 10, 30

Which TWO configuration adjustments must be implemented on Switch-Beta to resolve both issues? (Select TWO.)

  1. Reconfigure the native VLAN on interface GigabitEthernet 0/1 to VLAN 10.Answer
  2. Add VLAN 20 to the list of allowed VLANs on interface GigabitEthernet 0/1.Answer
  3. C
    Configure speed and duplex settings on interface GigabitEthernet 0/1 to auto-negotiate.
  4. D
    Lower the Spanning Tree Protocol (STP) bridge priority for VLAN 20 on Switch-Beta to 4096.

Answer

The administrator must reconfigure the native VLAN on interface GigabitEthernet 0/1 to VLAN 10 and add VLAN 20 to the allowed VLAN list on interface GigabitEthernet 0/1.
The correct solution requires two actions on Switch-Beta: modifying the native VLAN on interface GigabitEthernet 0/1 to VLAN 10, and adding VLAN 20 to the trunk interface allowed list. Changing the native VLAN fixes the native VLAN mismatch between Switch-Alpha (VLAN 10) and Switch-Beta (VLAN 30), preventing untagged frame leakage. Adding VLAN 20 to the allowed list permits VLAN 20 tagged frames to pass across the trunk interface.

Step-by-Step Solution

1
Analyze the syslog symptom regarding untagged traffic misdirection.
Identify that Switch-Alpha uses Native VLAN 10 while Switch-Beta uses Native VLAN 30 on the trunk link, creating a native VLAN mismatch that causes untagged VLAN 10 frames to drop into VLAN 30 on Switch-Beta.
802.1Q trunk ports do not tag frames belonging to the configured native VLAN. The receiving switch places untagged frames into its own configured native VLAN.
2
Analyze the VLAN 20 connectivity failure.
Determine that VLAN 20 is configured on Switch-Alpha's allowed trunk list (10, 20, 30) but is missing from Switch-Beta's allowed trunk list (10, 30).
Trunk interfaces prune and drop frames for VLANs that are not explicitly included in their allowed VLAN list.
3
Identify the required remedial commands on Switch-Beta.
Change the native VLAN on GigabitEthernet 0/1 to VLAN 10 and update the trunk allowed VLAN list to include VLAN 20.
These two steps align trunk parameter configurations on both ends, restoring VLAN 20 traffic flow and correcting untagged frame mapping.

Key Concept

802.1Q Trunk Parameter Matching (Native VLAN and Allowed VLAN List)
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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