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Difficulty: HardVLAN Configuration and 802.1Q Trunking

A network administrator is troubleshooting an IEEE 802.1Q trunk link configured between two enterprise switches, `SW-BUILDING-A` and `SW-BUILDING-B`, on interface `GigabitEthernet1/0/48`.

The administrator inspects the running configuration on both switch interfaces:

`SW-BUILDING-A# show running-config interface GigabitEthernet1/0/48`
text
interface GigabitEthernet1/0/48
switchport mode trunk
switchport trunk native vlan 10
switchport trunk allowed vlan 10,20,30

`SW-BUILDING-B# show running-config interface GigabitEthernet1/0/48`
text
interface GigabitEthernet1/0/48
switchport mode trunk
switchport trunk native vlan 20
switchport trunk allowed vlan 10,20,30

Based on these interface configurations, which of the following operational outcomes or symptoms will occur across this trunk link? (Select TWO.)

  1. Untagged traffic originating from VLAN 10 on SW-BUILDING-A will be placed into VLAN 20 upon ingress at SW-BUILDING-B.Answer
  2. System log messages indicating a native VLAN mismatch will be periodically generated by discovery protocols such as CDP or LLDP.Answer
  3. C
    Tagged frames belonging to VLAN 30 will be dropped immediately because the native VLAN mismatch disables the trunking protocol.
  4. D
    Hosts in VLAN 10 on SW-BUILDING-A will maintain unbroken Layer 2 broadcast domain connectivity with hosts in VLAN 10 on SW-BUILDING-B.

Answer

Untagged traffic originating from VLAN 10 on SW-BUILDING-A will be placed into VLAN 20 upon ingress at SW-BUILDING-B, and system log messages indicating a native VLAN mismatch will be periodically generated by protocols like CDP or LLDP.
In IEEE 802.1Q trunking, traffic belonging to the native VLAN is forwarded untagged. When SW-BUILDING-A transmits untagged frames for its native VLAN 10, SW-BUILDING-B receives them untagged and assigns them to its configured native VLAN 20. This causes cross-VLAN traffic leakage and breaks single-VLAN Layer 2 continuity. Furthermore, management protocols like CDP or LLDP periodically exchange interface parameters and issue native VLAN mismatch syslog notifications upon detecting mismatched native VLAN IDs.

Step-by-Step Solution

1
Analyze the 802.1Q native VLAN behavior on the sending switch (SW-BUILDING-A).
SW-BUILDING-A has `switchport trunk native vlan 10`. Frames originating on VLAN 10 are sent out GigabitEthernet1/0/48 untagged (without an 802.1Q header).
By standard 802.1Q specification, traffic matching the local native VLAN ID on a trunk interface is transmitted unencapsulated.
2
Analyze frame ingress processing on the receiving switch (SW-BUILDING-B).
SW-BUILDING-B has `switchport trunk native vlan 20`. Any untagged frame arriving on GigabitEthernet1/0/48 is implicitly assigned to VLAN 20.
Receiving switches associate untagged trunk ingress traffic with their own locally configured native VLAN.
3
Evaluate protocol monitoring behaviors across mismatched native VLAN trunks.
Link-layer discovery protocols (CDP/LLDP) detect the mismatch in PDU exchange fields and log persistent console/syslog errors.
Network operating systems use management protocols to validate trunk parameters and alert administrators to configuration errors.

Key Concept

IEEE 802.1Q Native VLAN Mismatch and Frame Tagging Dynamics
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Topics

VLAN Configuration and 802.1Q Trunking

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