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Difficulty: Very hardSwitching Concepts and VLANs

A network engineer is inspecting Layer 2 traffic behavior across two directly connected switches, Switch-A and Switch-B, linked via an 802.1Q trunk interface. Switch-A is configured with native VLAN 10 on its trunk port, while Switch-B is configured with native VLAN 20 on its corresponding trunk port. Allowed VLANs on both trunk ports include VLANs 10, 20, and 30. A host connected to an access port assigned to VLAN 10 on Switch-A generates an ARP broadcast request. Which TWO of the following statements accurately describe how the switches process, tag, and forward this frame across the trunk link?

  1. Switch-A transmits the ARP broadcast frame out of its trunk port without an 802.1Q VLAN header.Answer
  2. Switch-B assigns the incoming untagged ARP broadcast frame to VLAN 20 and floods it out all active access ports in VLAN 20.Answer
  3. C
    Switch-A appends an 802.1Q tag containing VLAN ID 10 because broadcast frames are mandatory-tagged across all trunk links regardless of native VLAN settings.
  4. D
    Switch-B rewrites the source MAC address and Layer 3 IP header of the frame to perform inter-VLAN routing between VLAN 10 and VLAN 20.
  5. E
    Switch-B drops the frame immediately upon receipt because untagged frames are disallowed on 802.1Q trunks configured with allowed VLAN lists.

Answer

The correct statements are that Switch-A transmits the ARP broadcast frame out of its trunk port without an 802.1Q VLAN header, and Switch-B receives the untagged frame and assigns it to VLAN 20, flooding it out all active access ports in VLAN 20.
Under IEEE 802.1Q specifications, a switch transmitting traffic belonging to its configured native VLAN over a trunk interface strips the 4-byte 802.1Q VLAN header, sending standard untagged Ethernet frames. On the receiving side, Switch-B associates any incoming untagged frame with its own configured native VLAN (VLAN 20). Consequently, the broadcast frame generated in VLAN 10 on Switch-A leaks directly into VLAN 20 on Switch-B and is flooded to all VLAN 20 access ports.

Step-by-Step Solution

1
Determine frame ingress and tagging logic at Switch-A
Host frame enters Switch-A on access VLAN 10. Switch-A prepares to forward the broadcast frame out trunk port Gi0/1. Because native VLAN on Switch-A's trunk is configured as VLAN 10, the 802.1Q tag is omitted (sent untagged).
IEEE 802.1Q standard requires frames belonging to the configured native VLAN of a trunk interface to be transmitted without a 4-byte 802.1Q tag.
2
Determine frame reception and classification logic at Switch-B
Switch-B receives an untagged Ethernet frame on its trunk interface Gi0/1. Switch-B evaluates its local trunk configuration, where native VLAN is set to 20. Switch-B implicitly assigns the untagged frame to VLAN 20.
A switch receiving untagged traffic on an 802.1Q trunk port automatically maps that traffic to its own locally configured native VLAN ID.
3
Analyze Layer 2 broadcast forwarding behavior
Once classified into VLAN 20 on Switch-B, the broadcast frame is flooded to all interfaces belonging to VLAN 20's broadcast domain.
Switches flood broadcast frames to all operational ports within the destination frame's assigned VLAN broadcast domain.

Key Concept

802.1Q Native VLAN Tagging Mechanics and Mismatch Traffic Leakage
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Topics

Switching Concepts and VLANs

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