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Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

A Cisco Catalyst switch interface configured as an 802.1Q trunk receives an incoming frame. Place the internal operational steps executed by the switch to process this frame into the correct chronological order from frame arrival to egress forwarding.

  1. 1Inspect the Ethernet frame header to check for the 802.1Q Tag Protocol Identifier (TPID 0x8100).
  2. 2Determine the VLAN association by extracting the 12-bit VLAN ID if tagged, or assigning the configured Native VLAN ID if untagged.
  3. 3Perform ingress filtering by verifying whether the determined VLAN ID exists in the interface's allowed VLAN list.
  4. 4Update the switch MAC address table by binding the frame's source MAC address to the ingress interface and determined VLAN ID.
  5. 5Forward or flood the frame out designated egress ports belonging to the matching VLAN forwarding domain.

Answer

The correct sequence for 802.1Q ingress frame processing is: 1) Inspect header for TPID (0x8100), 2) Extract tagged VLAN ID or assign configured Native VLAN ID if untagged, 3) Verify VLAN against the allowed VLAN list (ingress filtering), 4) Update MAC address table for the VLAN domain, 5) Forward or flood the frame to egress ports in that VLAN.
The correct operational sequence begins when an ingress frame arrives on an 802.1Q trunk. The switch ASIC first checks for the 802.1Q TPID (0x8100). Next, it extracts the 12-bit VLAN ID from tagged frames or assigns the configured native VLAN ID to untagged frames. After determining the VLAN, the switch applies ingress filtering to verify if that VLAN ID is on the trunk's allowed VLAN list. If permitted, it updates its MAC address table associating the source MAC with that VLAN and ingress port, and finally makes the egress forwarding or flooding decision within that VLAN context.

Step-by-Step Solution

1
Inspect the Ethernet header for 802.1Q encapsulation
Identifies if the frame contains a 4-byte 802.1Q header with EtherType 0x8100.
The switch must determine whether the frame is tagged before interpreting VLAN fields.
2
Derive the internal VLAN context
Extracts the 12-bit VID from the tag header, or maps untagged traffic to the native VLAN.
802.1Q rules dictate that untagged frames belong to the native VLAN, while tagged frames belong to their explicit VID.
3
Execute ingress VLAN pruning check (allowed list filtering)
Checks if the derived VLAN is permitted by the 'switchport trunk allowed vlan' configuration.
If the VLAN is pruned/disallowed, the switch drops the frame immediately prior to MAC table processing.
4
Learn source MAC address
Adds or updates the source MAC address entry in the CAM table for the derived VLAN context.
Dynamic MAC learning occurs per-VLAN after verifying ingress permission.
5
Perform egress forwarding decision
Forwards frame to egress destination interface(s) in the matching VLAN domain.
Layer 2 forwarding logic constrains broadcast and unicast traffic strictly within the matched VLAN boundary.

Key Concept

802.1Q Trunk Ingress Frame Processing Pipeline
Estimated Time:2m 0s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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