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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

A host in VLAN 20 sends an Ethernet frame to another host in VLAN 20 located on a remote switch across an IEEE 802.1Q trunk link. What is the correct chronological sequence of events for processing and forwarding this frame from the source access port to the destination access port?

  1. 1Switch 1 receives an untagged Ethernet frame from the source host on an access port assigned to VLAN 20.
  2. 2Switch 1 inserts a 4-byte 802.1Q header containing VLAN ID 20 into the frame.
  3. 3Switch 1 transmits the tagged Ethernet frame across the trunk link to Switch 2.
  4. 4Switch 2 receives the tagged frame on its trunk interface and identifies it as VLAN 20 traffic.
  5. 5Switch 2 removes the 4-byte 802.1Q tag from the frame header.
  6. 6Switch 2 delivers the original untagged Ethernet frame to the destination host on the VLAN 20 access port.

Answer

The correct chronological sequence begins with Switch 1 receiving the untagged frame on an access port, inserting a 4-byte 802.1Q tag for VLAN 20, transmitting the tagged frame over the trunk, Switch 2 receiving and identifying the tagged frame, Switch 2 stripping the 802.1Q header, and finally delivering the untagged frame to the destination host on the access port.
The correct sequence follows standard IEEE 802.1Q trunking operations: frames enter an access port untagged, are tagged with a 4-byte header containing the VLAN ID when forwarded out a trunk port for non-native VLANs, travel across the link, are read and stripped of the tag by the remote switch, and are finally delivered untagged to the destination host on an access port in the same VLAN.

Step-by-Step Solution

1
Identify frame ingress at the source switch
The source switch accepts an untagged frame on a standard VLAN 20 access port.
End-user hosts send standard Ethernet frames without 802.1Q tags.
2
Apply 802.1Q tagging prior to trunk egress
A 4-byte 802.1Q tag with VLAN ID 20 is inserted into the Ethernet header.
Trunk links require frame tagging to distinguish traffic belonging to non-native VLANs across switches.
3
Transmit the tagged frame across the trunk
The frame travels over the physical interswitch link.
The frame carries the VLAN 20 identifier across the shared trunk interface.
4
Process frame ingress at the receiving switch
Switch 2 reads the 802.1Q tag to determine the target VLAN.
The receiving switch relies on the tag to route the frame internally within the correct VLAN context.
5
De-encapsulate the 802.1Q frame
The 4-byte 802.1Q header is stripped from the frame.
Standard access ports must not send tagged frames to end hosts that do not support 802.1Q tagging.
6
Forward untagged frame to destination host
The original frame is delivered out the destination VLAN 20 access port.
The destination host receives standard Ethernet traffic identical to what was originally transmitted.

Key Concept

802.1Q Frame Tagging and Trunk Processing Lifecycle
Estimated Time:1m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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