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Vlan hopping

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

VLAN hopping is unauthorized or unintended traffic crossing from one VLAN into another VLAN.

VLAN hopping occurs when traffic crosses from one VLAN into another, commonly because trunk ports use different native VLANs. A native VLAN mismatch can misclassify untagged frames and create forwarding or security problems. section 12.3.2

Native VLAN mismatch

If SW1 uses VLAN 10 as its native VLAN and SW2 uses VLAN 30, SW1 sends a VLAN 10 frame untagged. SW2 assigns it to VLAN 30, so it cannot deliver the frame to a destination in VLAN 10.

Reduce the risk by:

  • Matching the native VLAN on both ends of every trunk. section 12.3.2
  • Using an unused VLAN, rather than VLAN 1, as the native VLAN; for example, VLAN 999. section 12.3.2

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

How deep it goes
Traces untagged-frame handling through a native VLAN mismatch and identifies preventive configuration practices.
Examples
Uses SW1 and SW2 with native VLANs 10 and 30 to show an untagged frame being assigned to the wrong VLAN.
What you can do after
Configure trunk ports with matching native VLANs and an unused native VLAN to reduce VLAN-hopping risk.
How it connects
Connects native VLANs with trunk ports, 802.1Q tagging, access ports, PVST+, Rapid-PVST+, and CDP.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.