VLAN 101: What are the benefits of using VLANs?

As VMware Administrators take a more active role in Network Design they begin to ask more questions regarding why, how, or when to configure specific network features.  Creating VLANs today has become a common practice, let’s find out why!

There are five primary reasons why VLANs are used today:

  • Cost
  • Security
  • Performance
  • Manageability
  • Availability


It seems as though everything we do in IT in someway revolves around cost savings.  We constantly hear industry terms such as TCO (Total Cost of Ownership) and ROI (Return on Investment).  Bound by budgetary constraints and the race to zero, reducing costs within an enterprise IT environment has become one of our primary responsibilities.

In the enterprise, rack-mounted Ethernet switches are typically available in 24 or 48 port configurations.  In the past, if you wanted to separate network traffic for the computers of multiple departments, say HR (Human Resources) and Sales, you would have to purchase two physical Ethernet switches.  If both departments had enough computers to occupy all of the ports on both physical Ethernet switches there wouldn’t be any cost savings associated with using VLANs.  However, what if HR had 10 computers and Sales had 12 computers?  Purchasing two 24-port Ethernet switches (One for each department) would waste 24 ports (assuming one uplink is used per switch).  By implementing two VLANs (VLAN ID: 101 – HR, VLAN ID: 102 – Sales) we can use a single switch and isolate each departments network traffic.


VLANs logically separate network traffic preventing devices from listening to any network traffic on other Virtual Local Area Networks.  They also offer additional security by VLAN device assignment.

There are three common methods used to assign a device (computer, PC, printer, etc.) to a VLAN:

  • Port based – A switch port is manually configured to be a member of a specific VLAN(s). Any device connected to this port will belong to the VLAN.  Physical security such as restricted access to the location of the physical switch is required.
  • Protocol based – The Layer-3 protocol being carried by the frame is used to determine VLAN membership, this method is not commonly used today.
  • MAC based – the VLAN membership is based on the MAC address of the device.  This method offers additional security at the cost of increased management.

It is important to note that VLAN Security relies on proper configuration.  An improperly configured environment exposes the customer to exploits such as VLAN Hopping, VTP Attacks, MAC Attacks, and PVLAN attacks.  See SANS Institute Virtual LAN Security: weaknesses and countermeasures and Redscan – Ten top threats to VLAN security for more details.


Performance is increased by reducing broadcast traffic, leveraging L3 switch capabilities to achieve wire-speed routing between VLANs, and applying VLAN-based QoS policies.

On an Ethernet network, broadcast traffic is traffic that must be processed by every device on the same network segment.  In order to process the traffic the NIC driver interrupts the host CPU.  As a result, CPU utilization would be higher on hosts in a network segment with a significant amount of broadcast traffic.  According to a Fluke Networks Whitepaper VLAN Best Practices, “An average number of broadcasts should be 30 broadcasts per second, or less. While no officially sanctioned quantity is specified in standards documents, field performance monitoring suggests that broadcasts should not exceed about 30 broadcasts per second.”

Common broadcast traffic includes: ARP, DHCP, Routing Protocols (RIP v1), NetBIOS/SMB, IPX/SPX (SAP Broadcasts).

Using a “sniffer” such as Wireshark, you can view broadcast traffic on your network. Ethernet has a MAC level broadcast address (ff:ff:ff:ff:ff:ff), and an IP-level broadcast address (

Today there are features natively built into operating systems and switches that help reduce some broadcast traffic.  For example, Windows hosts use an ARP cache.  “Each dynamic ARP cache entry has a potential lifetime of 10 minutes. New entries added to the cache are timestamped. If an entry is not reused within 2 minutes of being added, it expires and is removed from the ARP cache. If an entry is used, it receives two more minutes of lifetime. If an entry keeps getting used, it receives an additional two minutes of lifetime up to a maximum lifetime of 10 minutes.”Microsoft TechNet Entries in the ARP Table of an ESXi host remain for 1200 seconds (20-minutes).

Ethernet Switches use MAC Learning to reduce the amount of broadcast traffic. “MAC learning allows the Ethernet switch to learn the MAC addresses of the stations in the network to identify on which port to send the traffic. LAN switches normally keep a MAC learning table (or a bridge table) and a VLAN table. The MAC learning table associates the MACs/VLANs with a given port, and the VLAN table associates the port with a VLAN.” – Sam Halabi – Cisco Press: Metro Ethernet Services  Using the sh mac-address-table aging-time command on our Cisco Catalyst switch reveals a Global Aging Time of 300 seconds (5-minutes).

CPUs have become so powerful that the impact broadcast traffic has on a host is minimal today.

In order for frames to get from one VLAN to another, a Layer-3 device must route them. This device could be a traditional router, or a Layer-3 switch. Each router hop adds additional latency to the time it takes to get the frame from the sender to the receiver and can act as a bottleneck.  By using a Layer-3 switch, traffic never leaves the switch and could offer better performance.  Using a Layer-3 switch allows you to apply wire-speed routing between VLANs.

You can also use the QoS: Match VLAN feature to classify network traffic on the basis of the Layer 2 virtual local-area network (VLAN) identification number.  QoS—VLAN Tag-Based feature can then be leveraged to apply a single QoS policy, referred to as a VLAN-group policy, to a group of IEEE 802.1Q VLAN subinterfaces.


Let’s say you wanted to use of a single subnet ( for all of the printers at the Corporate Office.  In order to do so, each printer on the subnet would need to be in the same broadcast domain. This would require a dedicated switch for the printers, which wouldn’t be too challenging.  However, if the Corporate Office had five floors, you would now need a switch on each floor for just the printers. These switches would need to be connected to each other to extend the network to each floor. Using a VLAN would allow the printers to be connected to the same switches as other devices on the network.  Fewer switches = less management.

According to Cisco, VLANs also improve IT staff efficiency. “VLANs make it easier to manage the network because users with similar network requirements share the same VLAN. When a new switch is provisioned, all the policies and procedures already configured for the particular VLAN are implemented when the ports are assigned. It is also easy for the IT staff to identify the function of a VLAN by giving it an appropriate name.”  – Cisco Networking Academy Introduction to VLANs

Switch features such as VLAN Trunk Protocol (VTP), make it easy to distribute VLANs across a physical network environment.


The amount of broadcast traffic generated today is significantly less than it has been in the past.  Protocols such as IPX/SPX, RIP Version 1, and NetBIOS aren’t as common today.  Networking technologies such as vSphere NSX also reduce broadcast traffic through features such as ARP suppression. Furthermore, host CPUs have become so powerful that the additional task of handling broadcast traffic is insignificant host performance.

So why bother with VLANs?  Think “Failure Domain”  VLANs offer the ability to reduce the size your failure domain.  If a device has a damaged Network Interface Card (NIC) it may broadcast enough traffic to impact every host in the VLAN.  For example, if a desktop computer in the Sales VLAN had a bad NIC that had become “chatty” and created a storm of broadcast traffic, only the devices on the Sales VLAN would be impacted.

How do I configure a VLAN?

Before we get started we need to briefly define the difference between an “access” port, and a “trunk” port.

  • An access port can have only one VLAN configured on the interface; it can carry traffic for only one VLAN.  Traffic that reaches an access port is
  • A trunk port can have two or more VLANs configured on the interface; it can carry traffic for several VLANs simultaneously.

Now, let’s use our previous example where the HR and Sales departments are sharing a single 24-port Ethernet switch, ten (10) HR computers and twelve (12) Sales computers:

  • Ports 1-12 are connected to the ten (10) computers for the HR department
  • Ports 13-22 are connected to the twelve (12) computers for the Sales department
  • Port 24 is the “uplink” port for Internet Access

In order for the HR department computers to communicate with each other and the internet we will associate ports 1-12 and 24 with VLAN 101. Ports 1-12 would be designated are configured as “access” ports, while port 24 would be designated as a “trunk” port.

Next we will create a second VLAN by associating ports 13-22 and 24 with VLAN 102. In this case, ports 13-22 would be designated as “access” ports while port 24 would be designated as “trunk” port.

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