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Difficulty: Very hardInterswitch Connectivity and Trunking (802.1Q)

A network engineer connects two Cisco Catalyst switches. SwitchA interface GigabitEthernet0/1 is configured with 'switchport mode dynamic desirable', and SwitchB interface GigabitEthernet0/1 is configured with 'switchport mode dynamic auto'. Both switches use default native VLAN 1 configurations. Place the following operational events in the correct sequential order from the initiation of link negotiation to the transmission of native VLAN traffic.

  1. 1SwitchA transmits Dynamic Trunking Protocol (DTP) negotiation frames out interface GigabitEthernet0/1 actively proposing to establish a trunk link.
  2. 2SwitchB receives the incoming DTP proposal on GigabitEthernet0/1 and responds with a DTP agreement frame.
  3. 3Both switch interfaces successfully complete negotiation and transition their operational trunking state to 802.1Q.
  4. 4SwitchA receives an ingress broadcast frame assigned to VLAN 1 destined for SwitchB.
  5. 5SwitchA transmits the frame out GigabitEthernet0/1 across the trunk link without inserting a 4-byte 802.1Q VLAN tag.

Answer

The correct sequence begins with SwitchA sending active DTP frames, followed by SwitchB acknowledging the proposal from dynamic auto mode, both interfaces transitioning to operational 802.1Q trunking, SwitchA receiving ingress VLAN 1 traffic, and SwitchA forwarding that native VLAN traffic untagged across the trunk.
Dynamic desirable ports actively initiate DTP negotiation, causing dynamic auto ports to respond and agree. Once operational, data traffic matching the default native VLAN (VLAN 1) is forwarded across the 802.1Q trunk untagged.

Step-by-Step Solution

1
Initiate DTP Proposal
SwitchA sends DTP frames via Gi0/1.
'switchport mode dynamic desirable' actively initiates negotiation with the neighbor.
2
Respond to Negotiation
SwitchB receives DTP and responds positively.
'switchport mode dynamic auto' will not originate DTP proposals but willingly converts to a trunk if requested by a neighbor.
3
Establish Operational Trunk
Both Gi0/1 interfaces enter the trunking operational state using 802.1Q encapsulation.
A desirable-to-auto dynamic DTP pairing successfully negotiates a trunk.
4
Process Ingress Frame
SwitchA accepts an ingress frame belonging to VLAN 1 (native VLAN).
Data plane forwarding occurs after control plane negotiation is established.
5
Forward Across Trunk
SwitchA sends the frame across Gi0/1 untagged.
By default under 802.1Q, native VLAN traffic is transmitted without adding a 4-byte VLAN tag.

Key Concept

802.1Q Trunking DTP Negotiation Sequence and Native VLAN Tagging Mechanics
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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