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Using VLAN’s in Network Design
Kevin Colo
December, 2012
1. Background
To this day, end users still ask if VLANs (Virtual LANs) are a fundamentally secure technique for isolating
The answer: Yes.
VLAN hopping (the ability to gain access to a different VLAN than the one you are supposed to) was a large
security concern about 12 years ago. Today, these legacy exploits are well understood and are mitigated
using proper network device configuration.
The idea of using a separate physical switch for different security domains is no longer prevalent in modern
enterprise campus networks. At the enterprise edge, separate switches have been displaced by VLANs,
multi-context firewalls, and consolidated chassis implementations.
Today, when a “one switch per VLAN” design is used, it typically implies:
The “Outside” switch of an internet connection, and sometimes the “DMZ” switch. Even in this case,
it is now common to have multiple DMZs separated using VLANs.
A desire to do things, “the way we did it last time”.
A desire to not have equipment be audited (e.g., in-scope for PCI compliance)
Specific internal network policy that when researched, hasn’t been updated in many years.
VLANS are safe to use for network security isolation, but like anything else, network devices must be
configured correctly using standard best practices. There are many documentation references available to
assist in the proper configuration of network devices.
From RFC 4554:
Ethernet VLANs are quite commonly used in enterprise networks for the purposes of traffic segregation.
From Wikipedia:
There are two primary methods of VLAN hopping: switch spoofing and double tagging. Both attack vectors
can be easily mitigated with proper switchport configuration.
The V stands for Virtual
The use of VLANs ranges from the well-known LAN switches to firewalls, wireless APs, virtual switches in
virtualized environments, to the underlying customer isolation techniques used by Internet Service
Providers. Modern network designs would not be possible without the use of VLANs for traffic and security
segmentation. Service Providers offer VPLS, MPLS, and 802.1QnQ services that leverage VLANs (and in
the case of MPLS/VPLS, Virtual Routing/Forwarding VRF) for customer isolation.
Long before virtualization became a buzzword, VLANs were used as fundamental component of network
virtualization. 1 switch + 2 networks = virtualization. Virtualization technologies assume the consolidation of
physical hardware assets for a large number of well-known reasons (power, efficiency, management, etc.)
These reasons carry over to why VLANs are so popular and trusted as a network security isolation
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It took a bit of training and experience, but today’s IT departments have embraced the idea of taking one
piece of hardware, separating it into multiple logical domains, and leveraging it more efficiently.
Always follow the manufacturer’s configuration recommendations for all network devices. For instance,
mitigating VLAN hopping attacks on LAN switches typically comes down to:
1. Always use a dedicated VLAN ID for all trunk ports.
2. Disable unused ports and put them in an unused VLAN.
3. Do not use VLAN 1 for anything.
4. Disable auto-trunking on user facing ports (DTP off).
5. Explicitly configure trunking on infrastructure ports.
6. Use all tagged mode for the native VLAN on trunks.
7. Use PC voice VLAN access on phones that support it.
8. Use 802.1Q tag on all trunk ports.
Note: There are a number of OSI Layer 2 attacks not related to VLAN hopping. Specific mitigation
techniques are also available for these attacks.
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VLAN Use Examples
1.1 Campus
1.4 Firewalls
Access to distribution
Multi-context mode firewalls
Virtualization technologies that assume the
Transparent Mode Firewalls
consolidation of physical hardware:
Firewall trunk ports
VSS (Virtual Switching System)
vPC (Virtual Port Channel)
Intrusion Protection Systems
1.5 Service Provider
1.2 Data Center / Services
Switching (vPC)
Blade Chassis Switching
802.1Q-inQ Trunking Service (802.1ad)
Virtual switches
Load Balancers
1.6 Network Access Control
Remediation / Posture Assessment VLAN
1.3 Wireless LAN
Guest VLAN
Remote access points
Corporate user VLAN
Wireless LAN controller aggregation ports
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Understanding and Preventing Layer 2 Attacks in IPv4 Networks
Virtual LAN Security Best Practices
Enterprise Campus 3.0 Architecture: Overview and Framework
Cisco SAFE: A Security Blueprint for Enterprise Networks
Network Virtualization - Path Isolation Design Guide
Network Virtualization - Access Control Design Guide
Network Virtualization - Services Edge Design Guide
Cisco Wireless LAN Controller Configuration Guide, Release 7.2
Deploying Cisco 440X Series Wireless LAN Controllers
Secure Wireless Design Guide 1.0
Next Generation Enterprise MPLS VPN-Based MAN Design and Implementation Guide
RFC 2196 – Site Security Handbook
RFC 2401 - Security Architecture for the Internet Protocol
RFC 5517 - Cisco Systems' Private VLANs: Scalable Security in a Multi-Client Environment
RFC 4554 - Use of VLANs for IPv4-IPv6 Coexistence in Enterprise Networks
VLAN Security Guidelines - WatchGuard Technologies, Inc.
Secure Use of VLANs: An @stake Security Assessment
VLAN Hopping - Wikipedia
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