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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

A network engineer connects two Cisco Catalyst switches using interface GigabitEthernet0/1. Switch-1 is configured with the commands 'switchport mode dynamic desirable' and 'switchport trunk native vlan 10'. Switch-2 is configured with 'switchport mode dynamic auto' and 'switchport trunk native vlan 20'. Assuming default Dynamic Trunking Protocol (DTP) settings, which statement accurately describes the resulting operational state of this interswitch connection?

  1. An 802.1Q trunk will form successfully, but a native VLAN mismatch will occur, causing untagged traffic from VLAN 10 on Switch-1 to cross into VLAN 20 on Switch-2.Answer
  2. B
    The link will remain an access port in VLAN 1 because dynamic desirable and dynamic auto modes are incompatible and fail DTP negotiation.
  3. C
    Spanning Tree Protocol immediately places both interfaces into a blocking state because PortFast was automatically activated during trunk negotiation.
  4. D
    The trunk negotiation will succeed, but Switch-2 will silently drop all untagged traffic sent by Switch-1 without generating any logging or protocol warnings.

Answer

An 802.1Q trunk will form successfully, but a native VLAN mismatch will occur, causing untagged traffic from VLAN 10 on Switch-1 to cross into VLAN 20 on Switch-2.
When one switch interface is configured as 'dynamic desirable' and the opposite interface is 'dynamic auto', Dynamic Trunking Protocol (DTP) successfully negotiates the link into an operational 802.1Q trunk. However, because Switch-1 uses VLAN 10 as its native VLAN and Switch-2 uses VLAN 20, 802.1Q untagged frame behavior causes traffic sent on VLAN 10 from Switch-1 to arrive untagged and be placed into VLAN 20 on Switch-2. This native VLAN mismatch generates continuous syslog alerts via CDP.

Step-by-Step Solution

1
Evaluate DTP Negotiation Modes
Dynamic Desirable on Switch-1 actively sends DTP frames, while Dynamic Auto on Switch-2 listens and responds positively to DTP trunk requests.
DTP mode dynamic desirable paired with dynamic auto results in an operational trunk link.
2
Analyze Native VLAN Configuration
Switch-1 expects untagged frames to belong to VLAN 10, whereas Switch-2 expects untagged frames to belong to VLAN 20.
802.1Q trunks do not tag frames belonging to the native VLAN across the link header.
3
Determine Frame Delivery Behavior and Protocol Alerts
Untagged frames sent from VLAN 10 on Switch-1 travel across the trunk untagged and are ingested into VLAN 20 by Switch-2. Cisco Discovery Protocol (CDP) detects this discrepancy and generates syslog messages.
A native VLAN mismatch leads to VLAN hopping/leaking and triggers CDP native VLAN mismatch error logs.

Key Concept

DTP Negotiation Outcomes and 802.1Q Native VLAN Alignment
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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