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Difficulty: Very hardSwitching Concepts and VLANs

A network engineer is configuring an IEEE 802.1Q trunk connection between two Layer 2 enterprise switches, Switch-A and Switch-B. Switch-A has its trunk port configured with native VLAN 10, whereas Switch-B has its trunk port configured with native VLAN 20. Host 1 is attached to an access port on Switch-A assigned to VLAN 10, and Host 2 is attached to an access port on Switch-B assigned to VLAN 20. If Host 1 transmits an unencapsulated broadcast frame into Switch-A, what occurs when this traffic traverses the link to Switch-B and reaches Host 2?

  1. Switch-A transmits the frame untagged across the trunk. Switch-B receives the untagged frame, assigns it to its native VLAN 20, and forwards it to Host 2 without requiring a Layer 3 router.Answer
  2. B
    Switch-A drops the frame at the ingress access port because the frame cannot cross mismatched native VLANs without an active Spanning Tree Protocol (STP) root bridge resolving the conflict.
  3. C
    Switch-A encapsulates the frame with an 802.1Q header for VLAN 10. Switch-B inspects the tag, identifies a native VLAN mismatch, and drops the frame at its trunk interface.
  4. D
    Switch-A forwards the frame untagged, but Switch-B drops the frame because hosts in different VLANs can never communicate under any physical topology without Layer 3 inter-VLAN routing.

Answer

Switch-A transmits the frame untagged across the trunk. Switch-B receives the untagged frame, assigns it to its native VLAN 20, and forwards it to Host 2 without requiring a Layer 3 router.
Under IEEE 802.1Q trunking standards, frames assigned to the native VLAN of an egress trunk interface are transmitted untagged. Because Switch-A has native VLAN 10 configured, any frame from VLAN 10 traversing the trunk is sent without an 802.1Q tag. When Switch-B receives an untagged frame on its trunk port, it automatically attributes that frame to its own locally configured native VLAN (VLAN 20). Consequently, the broadcast frame is flooded to all ports in VLAN 20 on Switch-B, reaching Host 2 directly at Layer 2.

Step-by-Step Solution

1
Analyze Switch-A ingress and egress trunk behavior
Host 1 sends an unencapsulated frame into an access port on VLAN 10. When Switch-A forwards this frame out its trunk port, it compares the frame's VLAN (VLAN 10) to the trunk port's configured native VLAN (VLAN 10). Because they match, Switch-A transmits the frame untagged across the trunk link.
According to IEEE 802.1Q standards, traffic matching the local native VLAN of a trunk port is sent without an explicit 4-byte 802.1Q VLAN header.
2
Analyze Switch-B ingress trunk processing
Switch-B receives the untagged frame on its trunk port. Switch-B maps all incoming untagged frames received on a trunk interface to its own locally configured native VLAN, which is VLAN 20.
Trunk ports inspect incoming frames; if no 802.1Q tag is present, the switch implicitly tags/assigns the frame to its local native VLAN.
3
Analyze Layer 2 broadcast forwarding to destination host
Because the broadcast frame is now internally associated with VLAN 20 on Switch-B, Switch-B floods the frame out all ports belonging to VLAN 20 broadcast domain, delivering it directly to Host 2.
The native VLAN mismatch leaks Layer 2 traffic between VLAN 10 on Switch-A and VLAN 20 on Switch-B without requiring a Layer 3 routing device.

Key Concept

802.1Q Native VLAN Ingress/Egress Tagging and Traffic Leakage
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Topics

Switching Concepts and VLANs

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