Static and Dynamic VLAN Membership Explained

There are two ways to assign VLAN membership to a switchport: static and dynamic. In the static method, you manually add switch ports to VLANs. In the dynamic process, the switch automatically assigns appropriate VLANs to ports.

What is VLAN membership?

A VLAN is a group of devices that share broadcast messages. VLAN membership is an authorization to be part of the VLAN. By default, switches have only one VLAN, called VLAN-1. However, you can create and configure additional VLANs to limit broadcast messages and logically arrange devices in groups. After creating VLANs, you add the desired ports to them. There are two ways to do this: Static and Dynamic.

Static VLAN membership

In the static method, you manually add ports to VLANs. VLANs configured in this way are typically called port-based VLANs. The following image shows an 8-port switch. With the default configuration, all ports belong to the same broadcast domain.

Now, you want to break this network into two broadcast domains. You create two VLANs: VLAN-10 and VLAN-20. You enter the sub-configuration mode of ports and configure VLANs. You configure VLAN-10 on ports 1, 2, 3, and 4, and VLAN-20 on ports 5, 6, 7, and 8. It is an example of static VLAN membership.

Advantages of static VLAN membership

The main advantages of static VLAN membership are the following.

• Easy configuration
• Easy management
• Security

Easy configuration

Configuring a static VLAN membership is easy. To configure a static VLAN membership, you run only one command [switch(config-if)#switchport access vlan VLAN_ID] in the sub-interface configuration mode.

Easy management

Managing static VLAN membership is also straightforward. From the sub-interface configuration mode of the port, you can add, update, and remove VLAN membership.

Security

Since you manually add, update, and remove static VLAN membership on all ports, the static VLAN membership is more secure than the dynamic VLAN membership.

Availability

Static VLANs are available on almost all Cisco switches. You do not need high-end switches to use them. You can configure and use them on existing switches.

Disadvantages of static VLAN membership

The disadvantages of static VLANs are the following.

• Scalability
• Movability

Scalability

Static VLAN membership is not scalable. If your network is small, you can choose it because of its easy configuration. But if your network is big, you can not use it alone. For example, if your network has thousands of devices, assigning and managing static VLAN membership for these devices on switches will be a difficult task.

Movability

Static VLAN membership is not movable. If you move a PC from one switch port to another, you need to manually remove the VLAN membership from the current port and add it to the new port.

Dynamic VLAN membership

Dynamic VLAN membership works on a server-client model. In this model, a policy server called the VLAN membership policy server (VMPS) saves VLAN mapping information. VLAN mapping information contains VLAN IDs and MAC addresses of end devices.

All other switches act as VLAN client switches. They forward MAC addresses to the VMPS switch. The VMPS switch finds the VLAN ID associated with each MAC address and passes that ID to the VLAN client switch.

The following image shows a network that uses dynamic VLAN membership.

In the above network, when you add PC2 to the VLAN client switch's port 2, it forwards PC2's MAC address to the VMPS switch. VMPS switch finds the VLAN ID related to PC2's MAC address and provides that ID to the VLAN client switch. The VLAN client switch assigns the received VLAN ID to port 2. Now, you remove PC2 from port-2 and attach it to port-3. In this situation, the switch will take the following steps.

• It removes the configured VLAN on port-2 as soon as you remove the connected device.
• When you attach PC2 to port 3, it sends PC2's MAC address to the VMPS switch.
• VMPS switch finds the VLAN ID related to PC2's MAC address and provides it to the VLAN client switch.
• The VLAN client switch assigns the received VLAN ID to port-3.

Advantages of Dynamic VLAN membership

The advantages of dynamic VLAN membership are the following.

• Movability
• Centralized management
• Fast implementation

Movability

The main advantage of dynamic VLAN membership is mobility. If you move an end device from one switch port to another, the switch automatically updates the VLAN information on both ports.

Centralized management

Dynamic VLAN membership provides centralized management. You need to manage VLAN information only on the VMPS switch. Other switches automatically receive the updated information from the VMPS switch.

Fast implementation

Since VLAN client switches automatically implement dynamic VLAN membership, it is faster than static VLAN membership. In static VLAN membership, if you change VLAN information, you need to update that information on all ports manually. In dynamic VLAN membership, switches automatically update the information on all ports.

Disadvantages of Dynamic VLAN membership

The disadvantages of dynamic VLAN membership are the following.

• Complex configuration
• Add extra cost to the network

Complex configuration

The configuration of dynamic VLAN membership is complex. You need to map the MAC addresses of all end devices to VLANs on the VMPS switch. You also need to configure all other switches to get VLAN information from the VMPS switch.

Add extra cost to the network

You cannot use low-end Cisco switches as a VMPS server switch. You need a high-end Cisco switch, such as a Catalyst 6500 switch, to configure VMPS. High-end Cisco switches are costly.

Conclusion

Both static and dynamic VLAN membership models have their unique advantages and disadvantages, impacting network configuration and management. Static VLAN membership offers simplicity and ease of management, making it a suitable choice for smaller networks. Its straightforward configuration allows administrators to assign ports to VLANs manually with minimal effort. However, its lack of scalability and the need for manual updates can be cumbersome in larger environments where device mobility is common. On the other hand, dynamic VLAN membership enhances flexibility and scalability by automating the assignment of VLANs based on device MAC addresses. This model facilitates easier management in larger networks, as changes are made centrally on the VLAN membership policy server (VMPS) and automatically propagated to client switches. Despite its advantages, the reliance on centralized management can introduce complexity, and it may require more sophisticated infrastructure. Ultimately, the choice between static and dynamic VLAN membership will depend on the specific needs of the network, including its size, device mobility, and management preferences.