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Difficulty: MediumSwitching Concepts and VLANs

A network administrator connects two Layer 2 switches using an 802.1Q trunk link. Switch A has native VLAN 10 configured on its trunk port, while Switch B has native VLAN 20 configured on its trunk port. Host A, attached to an access port in VLAN 10 on Switch A, sends a broadcast frame. Switch A transmits this frame untagged across the trunk link. Which of the following best describes how Switch B processes this incoming untagged frame?

  1. Switch B assigns the incoming untagged frame to its local native VLAN (VLAN 20) and forwards it to all active access ports in VLAN 20.Answer
  2. B
    Switch B uses the physical trunk line to automatically forward the broadcast frame directly to ports assigned to VLAN 10 without needing Layer 3 routing.
  3. C
    Switch B inspects the Layer 3 IP header of the frame to determine its source subnet and appends a VLAN 10 tag before switching it.
  4. D
    Switch B automatically lowers its Spanning Tree Protocol (STP) priority value to elect Switch A as root bridge and block the trunk interface.

Answer

Switch B assigns the incoming untagged frame to its local native VLAN (VLAN 20) and forwards it to all active access ports in VLAN 20.
Under 802.1Q trunking specifications, frames belonging to the native VLAN are sent across the trunk untagged. When an untagged frame arrives at a receiving switch interface, that switch implicitly tags or assigns the frame to its own locally configured native VLAN. Because Switch A sends VLAN 10 traffic untagged and Switch B has native VLAN 20 configured, Switch B maps the incoming untagged frame into VLAN 20, causing cross-VLAN traffic leakage.

Step-by-Step Solution

1
Analyze how Switch A transmits VLAN 10 traffic across the trunk link.
Because VLAN 10 is configured as the native VLAN on Switch A's trunk port, Switch A strips the 802.1Q tag and sends the frame untagged across the link.
802.1Q trunking rules dictate that frames belonging to the port's native VLAN are sent without an explicit 802.1Q VLAN header.
2
Determine how Switch B interprets untagged ingress frames on its trunk port.
Switch B receives the untagged frame and associates it with its own configured native VLAN, which is VLAN 20.
A switch interface assigned an 802.1Q native VLAN treats any incoming frame without a VLAN tag as member traffic of that local native VLAN.
3
Evaluate the forwarding destination of the broadcast frame on Switch B.
Switch B floods the broadcast frame out all active interfaces belonging to VLAN 20.
Broadcast frames are flooded throughout their assigned Layer 2 broadcast domain.

Key Concept

802.1Q Native VLAN Operation and Mismatch Behavior
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Topics

Switching Concepts and VLANs

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