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Difficulty: MediumInterswitch Connectivity and Trunking (802.1Q)

An administrator is troubleshooting an interswitch link between SwitchA and SwitchB. Interface GigabitEthernet0/2 on SwitchA is configured with 'switchport mode dynamic desirable'. Interface GigabitEthernet0/2 on SwitchB is configured with 'switchport mode dynamic auto'. Trunking is successfully established, but the console displays periodic '%CDP-4-NATIVE_VLAN_MISMATCH' warning messages. What is the root cause of this error message?

  1. The two connected switch interfaces are configured with different native VLAN IDs.Answer
  2. B
    Dynamic Trunking Protocol (DTP) failed to form a trunk link because both interface modes are dynamic.
  3. C
    PortFast was globally enabled on SwitchA, causing 802.1Q tags to be stripped from native VLAN frames.
  4. D
    The switches negotiated conflicting encapsulation protocols due to CDP TLV advertisement failures.

Answer

The two connected switch interfaces are configured with different native VLAN IDs.
Cisco Discovery Protocol (CDP) periodically sends Layer 2 messages containing native VLAN information across switch links. When an 802.1Q trunk link is established between two switches, both sides must agree on the native VLAN ID. If one switch port is configured with native VLAN 1 and the other with native VLAN 99, CDP detects the mismatch and emits the %CDP-4-NATIVE_VLAN_MISMATCH console alert.

Step-by-Step Solution

1
Analyze DTP negotiation status between dynamic desirable and dynamic auto.
Dynamic desirable actively initiates DTP negotiation while dynamic auto responds, resulting in a successfully established 802.1Q trunk link.
Understanding DTP mode matrix confirms trunk operational status.
2
Analyze the syslog error message %CDP-4-NATIVE_VLAN_MISMATCH.
Cisco Discovery Protocol (CDP) exchanges Native VLAN Type-Length-Value (TLV) fields between neighboring switches across the trunk link.
When the native VLAN configured on one end of an 802.1Q trunk does not match the native VLAN on the opposing end, CDP triggers this specific warning log.

Key Concept

802.1Q Native VLAN Inconsistency and CDP Detection
Estimated Time:1m 15s
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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