Examkin

Question

Difficulty: EasyVLAN Configuration and 802.1Q Trunking

A network administrator is configuring an 802.1Q trunk connection between two switches. Devices on tagged VLANs communicate normally across the link, but untagged traffic sent from one switch arrives on an unexpected VLAN on the opposite switch. Which misconfiguration is the primary cause of this issue?

  1. A native VLAN mismatch exists between the trunk interfaces on the two switches.Answer
  2. B
    Inter-VLAN routing is disabled on the core Layer 3 switch.
  3. C
    The switchport access VLAN configuration is active instead of dynamic trunking.
  4. D
    The 802.1Q protocol automatically drops all untagged Ethernet frames by default.

Answer

A native VLAN mismatch exists between the trunk interfaces on the two switches.
The correct answer correctly identifies a native VLAN mismatch. Under the IEEE 802.1Q trunking specification, untagged frames transmitted across a trunk link belong to the sending switch's native VLAN. When received, the neighboring switch assigns those untagged frames to its own configured native VLAN. If these VLAN IDs differ between switches, traffic leaks from one VLAN into another without routing.

Step-by-Step Solution

1
Analyze trunk traffic behavior
Tagged traffic passes correctly, but untagged traffic is received on an unexpected VLAN.
802.1Q trunks transport tagged frames with an explicit 4-byte VLAN tag header, while untagged frames are implicitly assigned to the native VLAN configured on the receiving interface.
2
Identify the role of the Native VLAN
Recognize that mismatched native VLAN configurations cause untagged frame leakage.
If Switch-1 uses Native VLAN 10 and Switch-2 uses Native VLAN 20, untagged frames sent by Switch-1 will be interpreted as VLAN 20 traffic upon arrival at Switch-2.

Key Concept

802.1Q Native VLAN Operation
Estimated Time:45s
Rate this question

Topics

VLAN Configuration and 802.1Q Trunking

More questions from this topic