VLANs (Virtual Local Area Networks) are an important tool for improving network security by logically separating devices on a network. By dividing a physical network into separate broadcast domains, VLANs allow you to isolate devices and limit unnecessary traffic. Here is an in-depth look at how I can utilize VLANs to enhance security on my network.
What are VLANs and How Do They Work?
VLANs are used to partition a single physical switch or network into multiple logical networks. Devices in a VLAN can only communicate directly with other devices in the same VLAN. Communication between VLANs requires routing through a layer 3 device like a router or multilayer switch.
Some key characteristics of VLANs:
- VLANs are configured in the switches, not the network devices themselves.
- VLANs function at layer 2 of the OSI model – they use MAC addresses, not IP addresses.
- Each VLAN is a separate broadcast domain – devices hear broadcasts only from their own VLAN.
- VLANs allow you to logically segment devices even if they are plugged into the same physical switch.
VLANs are established by assigning each device to a VLAN based on parameters like port, MAC address, or policy. To communicate between VLANs, traffic must pass through a router.
VLAN Security Benefits
Here are some of the key benefits of using VLANs to improve network security:
Traffic Isolation
- Isolates broadcast traffic – Broadcasts are contained within the VLAN they originate from.
- Limits direct lateral communication – Devices can only directly communicate with other devices in the same VLAN.
- Stops ARP spoofing – ARP spoofing only works within a broadcast domain, so VLANs can prevent it between VLANs.
Access Control
- Segments departments or device types – Finance or HR departments can be isolated, or cameras/VoIP phones can be segmented from user devices.
- Simplifies access control rules – Access lists only need to permit/deny traffic between VLANs rather than all devices.
Chokepoint Control
- Limits network accessibility – Access to other VLANs must go through configured router interfaces. This creates a centralized policy enforcement point.
- Allows better monitoring – All inter-VLAN traffic can be monitored as it routes through the chokepoint.
Steps for Configuring VLANs
Here is an overview of the key steps involved in setting up VLANs for security:
1. Determine VLANs and Assignments
Decide how you want to logically separate devices – by department, device type, subnet, etc. Determine which devices belong to each VLAN.
2. Configure VLANs on Switches
Use the vlan command on switches to create new VLANs. Define VLAN IDs, names, and associated ports/devices. Configure trunk ports to carry inter-VLAN traffic.
3. Set Port Security
Use port security features like 802.1x or MAC address filters to control which devices can access each port and VLAN.
4. Configure Inter-VLAN Routing
Configure a router interface for each VLAN. Use access control lists (ACLs) to filter traffic between VLANs.
5. Verify Connectivity
Test connectivity between devices in the same VLAN and different VLANs. Verify they can/cannot communicate per the configuration.
VLAN Trunking Protocol (VTP)
VTP allows you to configure VLANs on one switch and have this information propagated to other switches:
- VTP carries VLAN configuration information to all switches
- This reduces manual configuration and risk of mismatches
- VTP has different modes including server, client, and transparent
VTP can simplify VLAN deployment, but should be implemented carefully as it presents a security risk if accessed maliciously.
Using VLANs to Isolate Security Zones
Some examples of how I might utilize VLANs to create isolated security zones:
- User VLAN – Contains employee computers, phones, printers. Default deny inbound from other VLANs.
- Production VLAN – For production equipment on the factory floor. Locked down and restricted communication.
- VoIP VLAN – For VoIP phones. Limits exposure from voice traffic vulnerabilities.
- Management VLAN – For admin servers and tools. Only management jump boxes can access.
Proper VLAN design is crucial for balancing security with business needs. Multiple overlapping VLANs can also be used to create more complex logical separations.
Conclusion
VLANs allow you to achieve logical network segmentation without needing to make physical topology changes. When planned and implemented correctly, VLANs can significantly improve network security through traffic isolation, access control, and creating defined chokepoints. Some key considerations are properly handling inter-VLAN routing, using port security, and limiting risky protocols like VTP. VLANs provide a powerful tool to design layered defenses into your network.
