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Difficulty: Very hardInterswitch Connectivity and Trunking (802.1Q)

Place the operational and negotiation events in the exact chronological sequence required for two Cisco Catalyst switches to establish a functional IEEE 802.1Q trunk link using Dynamic Trunking Protocol (DTP) following CLI configuration.

  1. 1The administrator configures switchport mode dynamic desirable on Switch-A and switchport mode dynamic auto on Switch-B.
  2. 2Switch-A generates and transmits DTP frames containing its negotiation state and supported trunk encapsulation types across the physical link.
  3. 3Switch-B receives the DTP frame, processes the desirable request, and responds to negotiate IEEE 802.1Q encapsulation.
  4. 4Both switch interfaces transition their operational mode to trunking and negotiate the operational encapsulation as IEEE 802.1Q.
  5. 5The switches begin forwarding traffic, inserting a 4-byte 802.1Q tag into Ethernet frames for non-native VLANs while passing native VLAN frames untagged.

Answer

The correct sequence begins with the CLI mode configuration on both switches, followed by Switch-A actively sending DTP request frames, Switch-B processing and agreeing to the trunking request, the operational transition of both interfaces to an active 802.1Q trunk, and finally the active forwarding and 4-byte tagging of frame traffic.
The correct order follows the standard Cisco IOS control and data plane progression: Configuration commands activate DTP on the interface -> The dynamic desirable interface actively sends DTP negotiation frames -> The dynamic auto interface receives the frame and agrees to form a trunk -> Both ports transition operationally to trunking status -> IEEE 802.1Q 4-byte frame tagging is applied to active traffic.

Step-by-Step Solution

1
Identify administrative configuration stage
Switch-A is set to dynamic desirable and Switch-B is set to dynamic auto.
Administrative commands must be applied to the interfaces before any control-plane negotiation protocols can generate messages.
2
Determine active protocol packet generation
Switch-A sends DTP frames actively across the link.
Dynamic desirable ports actively initiate negotiation by sending DTP frames, whereas dynamic auto ports only listen until prompted.
3
Analyze protocol response and parameter matching
Switch-B receives the DTP request and responds affirmatively.
Dynamic auto mode transitions to a trunk state upon receiving an active negotiation request from a desirable or trunking neighbor.
4
Establish operational port state
Both ports transition to operational status trunking with 802.1Q encapsulation.
Once parameters are agreed upon, hardware state registers update the port state from access to trunking.
5
Apply data-plane framing rules
Ethernet frames traversing the link receive a 4-byte 802.1Q VLAN header, except for frames belonging to the native VLAN.
Data forwarding and 802.1Q frame encapsulation occur only after trunk negotiation and port state activation are fully finalized.

Key Concept

802.1Q Trunking & DTP Negotiation Sequence
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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