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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network engineer configures interface GigabitEthernet0/1 on Switch-1 with the commands `switchport mode trunk` and `switchport nonegotiate`. The connected interface GigabitEthernet0/1 on Switch-2 remains in its default configuration of `switchport mode dynamic auto`. What is the resulting operational state of interface GigabitEthernet0/1 on Switch-2?

  1. It operates as an access port because it does not receive Dynamic Trunking Protocol (DTP) frames from Switch-1 to trigger trunk negotiation.Answer
  2. B
    It automatically negotiates an 802.1Q trunk because CDP detects that Switch-1 has trunking enabled on the interface.
  3. C
    It generates continuous native VLAN mismatch error messages because DTP is disabled on Switch-1.
  4. D
    It enables PortFast on the trunk link to establish forwarding without requiring DTP frames.

Answer

Interface GigabitEthernet0/1 on Switch-2 operates as an access port because `switchport nonegotiate` on Switch-1 suppresses DTP frames, leaving Switch-2 in its default passive access state.
The command `switchport nonegotiate` turns off DTP frame generation on Switch-1. Because interface GigabitEthernet0/1 on Switch-2 is set to `dynamic auto`, it relies on receiving incoming DTP negotiation frames to transition to a trunking state. Receiving no DTP frames from Switch-1, Switch-2 stays in its default operational mode of access, resulting in a misconfiguration where Switch-1 trunks while Switch-2 acts as an access port.

Step-by-Step Solution

1
Analyze the DTP configuration on Switch-1
Switch-1 is configured statically as a trunk (`switchport mode trunk`) and DTP frame transmission is disabled using `switchport nonegotiate`.
The `nonegotiate` command stops the interface from sending DTP frames to neighboring devices.
2
Analyze the DTP operational behavior on Switch-2
Switch-2 is in `switchport mode dynamic auto`, which means it will form a trunk only if it receives a DTP request frame (desirable or trunk mode) from the remote switch.
Dynamic auto interfaces never actively originate DTP negotiation requests; they remain passive.
3
Determine the resulting link operational state
Because Switch-1 sends no DTP frames, Switch-2 never receives the signal to become a trunk and defaults to an operational access port.
A link connecting a static trunk with `nonegotiate` to a `dynamic auto` interface creates a mode mismatch where one side is trunking and the other is an access port.

Key Concept

802.1Q Trunking DTP Negotiation and Nonegotiate Interaction
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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