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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An engineer connects two Cisco Catalyst switches using their GigabitEthernet0/1 interfaces. Interface GigabitEthernet0/1 on Switch-1 is configured with `switchport mode dynamic auto`. Interface GigabitEthernet0/1 on Switch-2 is configured with `switchport mode trunk` followed by `switchport nonegotiate`. What is the resulting operational state of the GigabitEthernet0/1 interface on Switch-1?

  1. It operates as an access port because dynamic trunking negotiation frames are suppressed by Switch-2.Answer
  2. B
    It operates as an 802.1Q trunk port because Switch-2 actively signals its trunking status through IEEE 802.1Q header tagging.
  3. C
    It transitions into an err-disabled state due to a Dynamic Trunking Protocol parameter mismatch between the switches.
  4. D
    It enables Spanning Tree PortFast automatically to bypass the trunk negotiation timeout.

Answer

The interface on Switch-1 operates as an access port because dynamic trunking negotiation frames are suppressed by Switch-2.
When an interface is in dynamic auto mode, it does not initiate DTP negotiation; it only responds to incoming DTP frames. Since the remote switch interface is configured with 'switchport nonegotiate', no DTP packets are sent across the link. Consequently, the local interface remains in its default access operational state.

Step-by-Step Solution

1
Analyze Switch-1 DTP configuration
Switch-1 is configured as 'dynamic auto', meaning it will operate as an access port unless it actively receives DTP desirable or trunk negotiation frames.
Ports in dynamic auto mode wait passively for DTP proposals from the neighboring switch.
2
Analyze Switch-2 DTP configuration
Switch-2 has 'switchport nonegotiate' applied, which completely stops the interface from transmitting DTP frames.
The 'switchport nonegotiate' command disables DTP frame generation on static trunk or access ports.
3
Determine the resulting link operational state on Switch-1
Because Switch-1 receives no DTP negotiation frames, it never transitions to trunk mode and remains operating as an access port.
Without receiving incoming DTP frames, a dynamic auto interface defaults to operational access mode.

Key Concept

DTP Mode Negotiation and Nonegotiate Behavior
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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