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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator is troubleshooting an 802.1Q trunk link between Switch-A and Switch-B. Host computers on VLAN 20 attached to Switch-B are unable to communicate with the default gateway located on Switch-A. Inspection of the trunk configuration reveals two issues: Switch-A is configured with native VLAN 20 while Switch-B is configured with native VLAN 1, and VLAN 20 is currently missing from the allowed VLAN list on Switch-B's trunk interface. Which TWO of the following statements accurately explain the technical causes of the communication failure in this setup? (Select TWO.)

  1. Untagged frames transmitted from VLAN 20 on Switch-A will be associated with VLAN 1 when received by Switch-B.Answer
  2. Tagged frames for VLAN 20 sent from Switch-A will be dropped by Switch-B's trunk interface.Answer
  3. C
    Hosts on VLAN 20 cannot reach the gateway because 802.1Q trunking disables Layer 2 forwarding when devices reside in different physical switches.
  4. D
    The connectivity failure occurs because setting the STP bridge priority to 65535 on Switch-A forces it to block all trunk ports for VLAN 20.

Answer

The two correct technical causes are that untagged frames transmitted from VLAN 20 on Switch-A will be associated with VLAN 1 when received by Switch-B, and tagged frames for VLAN 20 sent from Switch-A will be dropped by Switch-B's trunk interface.
The issue stems from two distinct trunking misconfigurations. First, a native VLAN mismatch (VLAN 20 on Switch-A vs. VLAN 1 on Switch-B) causes untagged frames sent from VLAN 20 on Switch-A to be incorrectly assigned to VLAN 1 upon arrival at Switch-B. Second, because VLAN 20 is missing from Switch-B's allowed VLAN list on the trunk, any tagged VLAN 20 frames arriving at or traversing Switch-B are dropped.

Step-by-Step Solution

1
Analyze the impact of mismatched native VLAN configurations across the 802.1Q trunk link.
Switch-A transmits VLAN 20 traffic untagged because VLAN 20 is configured as its native VLAN. Switch-B receives untagged traffic and places it into VLAN 1 (its configured native VLAN), creating a native VLAN mismatch error and misdirecting traffic.
802.1Q trunks do not tag frames belonging to the configured native VLAN.
2
Evaluate the impact of the allowed VLAN list configuration on Switch-B.
Any tagged frames sent for VLAN 20 that reach Switch-B will be filtered out and dropped at the trunk interface because VLAN 20 is absent from Switch-B's allowed list.
Trunk interfaces only prune or permit traffic explicitly matched in their configured allowed VLAN list.

Key Concept

802.1Q Trunking Native VLAN Mismatch and Allowed List Filtering
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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