Examkin

Question

Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An administrator notices CDP log messages indicating a native VLAN mismatch on an 802.1Q trunk link between two switches. Switch-1 is configured with native VLAN 99 on interface GigabitEthernet0/1, while Switch-2 uses the default native VLAN 1 on interface GigabitEthernet0/1. If a host in VLAN 99 on Switch-1 transmits untagged frames across the trunk link, how will Switch-2 process this traffic?

  1. Switch-2 receives the untagged frames and forwards them into VLAN 1, leading to cross-VLAN traffic leakage.Answer
  2. B
    Switch-2 automatically drops the untagged frames because 802.1Q encapsulation requires all frames on a trunk link to carry a 4-byte VLAN tag header.
  3. C
    CDP detects the mismatch and immediately places interface GigabitEthernet0/1 on both switches into the err-disabled operational state.
  4. D
    Spanning Tree Protocol automatically enables PortFast on interface GigabitEthernet0/1 to bypass trunk tag processing for mismatched traffic.

Answer

Switch-2 receives the untagged frames and forwards them into VLAN 1, leading to cross-VLAN traffic leakage.
Under IEEE 802.1Q trunking rules, frames belonging to the native VLAN are sent untagged across the link. When a receiving switch receives untagged frames, it automatically maps them to its own configured native VLAN. Consequently, traffic originating from VLAN 99 on Switch-1 will be placed into VLAN 1 on Switch-2, resulting in unintended cross-VLAN traffic leakage.

Step-by-Step Solution

1
Analyze how 802.1Q handles native VLAN traffic.
Frames assigned to the native VLAN of a trunk interface are transmitted across the physical link without an added 802.1Q 4-byte header tag.
802.1Q trunking specifies that native VLAN traffic remains untagged for backward compatibility with legacy non-trunking devices.
2
Evaluate the behavior of the receiving switch when native VLAN settings are mismatched.
Switch-2 receives untagged frames from Switch-1 (sent from VLAN 99) and places them into Switch-2's configured native VLAN (VLAN 1).
A receiving switch maps any incoming untagged frame directly to its own configured native VLAN.
3
Determine the operational security and switching impact.
Traffic leaks across VLAN boundaries between VLAN 99 on Switch-1 and VLAN 1 on Switch-2, creating a security risk and potential connectivity issues.
Mismatching native VLANs breaks VLAN isolation for untagged traffic without bringing the physical link down.

Key Concept

802.1Q Native VLAN Tagging Mechanics and Traffic Leakage
Rate this question

Topics

Interswitch Connectivity and Trunking (802.1Q)

More questions from this topic