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Native vlan mismatch

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

A native VLAN mismatch is a trunk-link misconfiguration in which the two connected ports assign different VLANs to untagged traffic.

A native VLAN mismatch occurs when connected trunk ports use different native VLANs. Because native-VLAN traffic crosses an 802.1Q trunk untagged, each switch can classify the same frame into a different VLAN. section 12.3.2

SW1 uses VLAN 10; SW2 uses VLAN 30

SW1 sends a VLAN 10 frame untagged because VLAN 10 is native on its trunk port. SW2 classifies that untagged frame as VLAN 30, so it cannot forward the frame to a destination in VLAN 10. Matching the native VLAN on both ends prevents this misconfiguration. section 12.3.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Traces untagged frames through mismatched trunk ports and stops short of detailed spanning-tree operation.
Examples
Uses SW1 and SW2 trunk ports with native VLANs 10 and 30 to demonstrate failed forwarding.
What it assumes
Assumes basic VLAN and trunk knowledge while establishing tagged versus untagged frame behavior.
What you can do after
Diagnose mismatches by comparing each trunk port’s native VLAN and configure both ends with the same value.
How it connects
Connects trunk ports, 802.1Q tagging, access ports, VLAN hopping, PVST+, Rapid-PVST+, and Cisco Discovery Protocol.
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Related concepts

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