What Is VLAN Hopping?

Updated on January 15, 2025

VLANs are a common tool in network administration used to segment traffic, improve performance, and boost security. However, they are not foolproof and can be exploited. This blog explains VLAN hopping—what it is, how it works, the risks it poses, and practical steps IT professionals can take to prevent it.

What Is VLAN Hopping?

VLAN hopping refers to a malicious technique where an attacker sends traffic across VLANs, bypassing the network segmentation created by administrators. Normally, VLANs ensure that devices on one VLAN cannot communicate directly with devices on another VLAN unless explicitly allowed. VLAN hopping undermines this isolation, granting attackers unauthorized access to other areas of the network.

To understand its significance, consider why VLANs are used:

• Improved Performance: By breaking down a large network into smaller segments, VLANs reduce congestion and improve efficiency.
• Enhanced Security: VLANs restrict traffic between segments, reducing the risk of unauthorized access to sensitive network resources.

By bypassing these controls, VLAN hopping exposes networks to serious threats like data breaches, unauthorized access, and the compromise of sensitive systems.

Techniques Used in VLAN Hopping

There are two primary techniques attackers use to execute VLAN hopping:

1. Double-Tagging Attacks

Double-tagging exploits the standard 802.1Q tagging protocol, which allows Ethernet frames to carry VLAN information.

How It Works:

• The attacker sends a specially crafted packet with two VLAN tags.
• The outer tag corresponds to the attacker’s current VLAN, while the inner tag is the VLAN ID the attacker wants to target.
• The first switch (usually the one connected to the attacker’s system) removes the outer tag, assuming it should forward the packet. However, it leaves the second tag intact.
• Once the packet reaches the next switch on the path, it reads the remaining tag, mistakenly forwarding the packet to the targeted VLAN.

Why It Happens:

Double-tagging takes advantage of switches that strip off only the first VLAN tag while forwarding packets.

Limitations:

• This attack only works in the direction of the trunk port.
• It relies on unidirectional forwarding, meaning the attacker cannot easily receive responses from the victim VLAN.

2. Switch Spoofing

Switch spoofing is another common VLAN hopping technique that exploits misconfigured trunk ports.

How It Works:

• The attacker pretends to be a switch by initiating Dynamic Trunking Protocol (DTP) negotiation with a legitimate switch.
• Upon successful negotiation, the legitimate switch configures the attacker’s port as a trunk port.
• The trunk port allows the attacker to receive and send traffic to any VLAN on the network.

Why It Happens:

This attack works due to switches being configured to dynamically negotiate trunking, a feature meant to simplify network management.

Risks:

Unlike double-tagging, switch spoofing can enable two-way communication, making it more dangerous.

Security Implications of VLAN Hopping

VLAN hopping poses a significant threat to enterprise networks. Here are the main risks:

1. Data Exposure Across VLANs

The primary purpose of VLANs is to restrict access between different network segments. VLAN hopping effectively circumvents this restriction, increasing the risk of data leakage across VLANs.

2. Unauthorized Access

Attackers can gain unauthorized access to systems and applications housed on other VLANs. For example, an attacker on a guest VLAN could potentially access a corporate VLAN containing sensitive data or resources.

3. Undermining Network Isolation

VLAN hopping compromises the principle of network isolation, making it easier for attackers to pivot through the network and expand their attack surface.

How to Mitigate VLAN Hopping Attacks

Preventing VLAN hopping requires network administrators to implement robust security measures. Here are some best practices:

1. Configure Trunk Ports Properly

• Disable Dynamic Trunking Protocol (DTP) on all ports that do not require trunking. Trunk ports should only be enabled on trusted interfaces.
• Manually configure which VLANs are allowed on each trunk port using VLAN pruning.

2. Disable Unused Ports

• Disable any unused switch ports to prevent attackers from connecting to them.
• Place unused ports in an isolated VLAN.

3. Harden VLAN Tagging

• Use dedicated VLAN IDs for trunk links and ensure these IDs are not used for access VLANs.
• Implement secure VLAN ID mapping to prevent double-tagging exploits.

4. Use Access Control Lists (ACLs)

• Deploy ACLs to filter traffic between VLANs. Strictly limit which devices and services can communicate across VLAN boundaries.

5. Enable Dynamic ARP Inspection (DAI)

• This feature inspects ARP packets and verifies their legitimacy, preventing attackers from bypassing VLAN restrictions by spoofing ARP packets.

6. Implement Private VLANs

• Private VLANs limit communication between devices on the same VLAN, adding an extra layer of isolation.

7. Monitor and Audit

• Regularly audit network configurations to ensure all ports, VLAN settings, and ACLs align with security best practices.
• Monitor network traffic for anomalies that may indicate VLAN hopping attempts.

Appendix: Key Terms

• VLAN Hopping: The unauthorized ability to send traffic between VLANs, bypassing network segmentation.
• 802.1Q Tagging: The Ethernet standard that allows VLAN identification through tagging.
• Double-Tagging Attack: A technique where attackers insert two VLAN tags into a packet to bypass VLAN boundaries.
• Switch Spoofing: An attack where an attacker impersonates a switch to gain trunk port access.
• Access Control List (ACL): A set of rules used to allow or deny traffic between different network segments.
• Trunk Port: A switch port configured to carry traffic for multiple VLANs.