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Cisco Networking Academy Switched Networks Companion Guide: VLANs

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 25, 2014.

Chapter Description

This chapter covers how to configure, manage, and troubleshoot VLANs and VLAN trunks. It also examines security considerations and strategies relating to VLANs and trunks, and best practices for VLAN design.

VLAN Segmentation (3.1)

LAN switches and VLANs go hand in hand. When you look at the configuration of a router, you do not see references to VLANs; however, when you look at the configuration of a switch, you see frequent references to VLANs. Modern switches are structured around VLANs. VLANs are to switches as networks are to routers. Almost everything you do on a switch relates to VLANs. So, to a large extent, learning about switching is learning about VLANs. The day in the future when every port on every switch is on a separate Layer 3 network is the day that VLANs are no longer necessary—the need for VLANs is tied to the need to put multiple switch ports in one broadcast domain (in one VLAN).

Overview of VLANs (3.1.1)

This section provides a high-level introduction to VLANs, which sets the stage for the chapter.

VLAN Definitions (

Within a switched internetwork, VLANs provide segmentation and organizational flexibility. VLANs provide a way to group devices within a LAN. A group of devices within a VLAN communicate as if they were attached to the same wire. VLANs are based on logical connections, instead of physical connections.

VLANs allow an administrator to segment networks based on factors such as function, project team, or application, without regard for the physical location of the user or device, as seen in Figure 3-1. Devices within a VLAN act as if they are in their own independent network, even if they share a common infrastructure with other VLANs. Any switch port can belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and flooded only to end stations within the VLAN where the packets are sourced. Each VLAN is considered a separate logical network, and packets destined for stations that do not belong to the VLAN must be forwarded through a device that supports routing.

Figure 3-1

Figure 3-1 Defining VLAN Groups

A VLAN creates a logical broadcast domain that can span multiple physical LAN segments. VLANs improve network performance by separating large broadcast domains into smaller ones. If a device in one VLAN sends a broadcast Ethernet frame, all devices in the VLAN receive the frame, but devices in other VLANs do not.

VLANs enable the implementation of access and security policies according to specific groupings of users. Each switch port can be assigned to only one VLAN (with the exception of a port connected to an IP phone or to another switch).

Benefits of VLANs (

User productivity and network adaptability are important for business growth and success. VLANs make it easier to design a network to support the goals of an organization. The primary benefits of using VLANs are as follows:

  • Security: Groups that have sensitive data are separated from the rest of the network, decreasing the chances of confidential information breaches. As shown in Figure 3-2, faculty computers are on VLAN 10 and completely separated from student and guest data traffic.

    Figure 3-2

    Figure 3-2 Benefits of VLANs

  • Cost reduction: Cost savings result from reduced need for expensive network upgrades and more efficient use of existing bandwidth and uplinks.
  • Better performance: Dividing flat Layer 2 networks into multiple logical workgroups (broadcast domains) reduces unnecessary traffic on the network and boosts performance.
  • Shrink broadcast domains: Dividing a network into VLANs reduces the number of devices in the broadcast domain. As shown in Figure 3-2, there are six computers on this network but there are three broadcast domains: Faculty, Student, and Guest.
  • Improved 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. In Figure 3-2, for easy identification, VLAN 10 has been named “Faculty,” VLAN 20 is named “Student,” and VLAN 30 “Guest.”
  • Simpler project and application management: VLANs aggregate users and network devices to support business or geographic requirements. Having separate functions makes managing a project or working with a specialized application easier; an example of such an application is an e-learning development platform for faculty.

Each VLAN in a switched network corresponds to an IP network; therefore, VLAN design must take into consideration the implementation of a hierarchical network-addressing scheme. Hierarchical network addressing means that IP network numbers are applied to network segments or VLANs in an orderly fashion that takes the network as a whole into consideration. Blocks of contiguous network addresses are reserved for and configured on devices in a specific area of the network, as shown in Figure 3-2.

Types of VLANs (

There are a number of distinct types of VLANs used in modern networks. Some VLAN types are defined by traffic classes. Other types of VLANs are defined by the specific function that they serve.


A data VLAN is a VLAN that is configured to carry user-generated traffic. A VLAN carrying voice or management traffic would not be a data VLAN. It is common practice to separate voice and management traffic from data traffic. A data VLAN is sometimes referred to as a user VLAN. Data VLANs are used to separate the network into groups of users or devices.

Default VLAN

All switch ports become a part of the default VLAN after the initial bootup of a switch loading the default configuration. Switch ports that participate in the default VLAN are part of the same broadcast domain. This allows any device connected to any switch port to communicate with other devices on other switch ports. The default VLAN for Cisco switches is VLAN 1. In Example 3-1, the show vlan brief command was issued on a switch running the default configuration. Notice that all ports are assigned to VLAN 1 by default.

VLAN 1 has all the features of any VLAN, except it cannot be renamed or deleted. By default, all Layer 2 control traffic is associated with VLAN 1.

Example 3-1 Default VLAN Configuration

Switch# show vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- --------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                Fa0/21, Fa0/22, Fa0/23, Fa0/24
                                                Gi0/1, Gi0/2
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup
Native VLAN

A native VLAN is assigned to an 802.1Q trunk port. Trunk ports are the links between switches that support the transmission of traffic associated with more than one VLAN. An 802.1Q trunk port supports traffic coming from many VLANs (tagged traffic), as well as traffic that does not come from a VLAN (untagged traffic). Tagged traffic refers to traffic that has a 4-byte tag inserted within the original Ethernet frame header, specifying the VLAN to which the frame belongs. The 802.1Q trunk port places untagged traffic on the native VLAN, which by default is VLAN 1.

Native VLANs are defined in the IEEE 802.1Q specification to maintain backward compatibility with untagged traffic common to legacy LAN scenarios. A native VLAN serves as a common identifier on opposite ends of a trunk link.

It is a best practice to configure the native VLAN as an unused VLAN, distinct from VLAN 1 and other VLANs. In fact, it is not unusual to dedicate a fixed VLAN to serve the role of the native VLAN for all trunk ports in the switched domain.

Management VLAN

A management VLAN is any VLAN configured to access the management capabilities of a switch. VLAN 1 is the management VLAN by default. To create the management VLAN, the switch virtual interface (SVI) of that VLAN is assigned an IP address and subnet mask, allowing the switch to be managed through HTTP, Telnet, SSH, or SNMP. Because the out-of-the-box configuration of a Cisco switch has VLAN 1 as the default VLAN, VLAN 1 would be a bad choice for the management VLAN.

In the past, the management VLAN for a 2960 switch was the only active SVI. On 15.x versions of the Cisco IOS for Catalyst 2960 Series switches, it is possible to have more than one active SVI. With Cisco IOS Release 15.x, the particular active SVI assigned for remote management must be documented. While theoretically a switch can have more than one management VLAN, having more than one increases exposure to network attacks.

In Example 3-1, all ports are currently assigned to the default VLAN 1. No native VLAN is explicitly assigned and no other VLANs are active; therefore the network is designed with the native VLAN the same as the management VLAN. This is considered a security risk.

Voice VLANs (

A separate VLAN is needed to support Voice over IP (VoIP). VoIP traffic requires

  • Assured bandwidth to ensure voice quality
  • Transmission priority over other types of network traffic
  • Ability to be routed around congested areas on the network
  • Delay of less than 150 ms across the network

To meet these requirements, the entire network has to be designed to support VoIP. The details of how to configure a network to support VoIP are beyond the scope of this course, but it is useful to summarize how a voice VLAN works between a switch, a Cisco IP Phone, and a computer.

In Figure 3-3, VLAN 150 is designed to carry voice traffic. The student computer PC5 is attached to the Cisco IP Phone, and the phone is attached to switch S3. PC5 is in VLAN 20, which is used for student data.

Figure 3-3

Figure 3-3 Voice VLAN

VLANs in a Multiswitch Environment (3.1.2)

VLAN trunks are the connections in switched networks upon which all control traffic is transmitted and received. VLAN trunks carry data traffic for all VLANs in the switched network, unless restricted manually or with a pruning mechanism. Switches are interconnected with VLAN trunks. This section describes VLAN trunks.

VLAN Trunks (

A trunk is a point-to-point link between two network devices that carries more than one VLAN. A VLAN trunk extends VLANs across an entire network. Cisco supports IEEE 802.1Q for coordinating trunks on Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet interfaces.

VLANs would not be very useful without VLAN trunks. VLAN trunks allow all VLAN traffic to propagate between switches so that devices that are in the same VLAN, but connected to different switches, can communicate without the intervention of a router.

A VLAN trunk does not belong to a specific VLAN; rather, it is a conduit for multiple VLANs between switches and routers. A trunk could also be used between a network device and server or other device that is equipped with an appropriate 802.1Q-capable NIC. By default, on a Cisco Catalyst switch, all VLANs are supported on a trunk port.

In Figure 3-4, the links between switches S1 and S2, and S1 and S3, are configured to transmit traffic coming from VLANs 10, 20, 30, and 99 across the network. This network could not function without VLAN trunks.

Figure 3-4

Figure 3-4 VLAN Trunks

Controlling Broadcast Domains with VLANs (

The behavior of broadcasts is affected by the presence of a switch. An ingress broadcast frame on a switch will only be forwarded out ports identified with the VLAN with which the frame is associated.

Network Without VLANs

In normal operation, when a switch receives a broadcast frame on one of its ports, it forwards the frame out all other ports except the port where the broadcast was received. In Figure 3-5, the entire network is configured in the same subnet ( and no VLANs are configured. As a result, when the faculty computer (PC1) sends out a broadcast frame, switch S2 sends that broadcast frame out all of its ports. Eventually the entire network receives the broadcast because the network is one broadcast domain.

Figure 3-05

Figure 3-5 VLAN Trunks

Network with VLANs

As shown in Figure 3-6, the network has been segmented using two VLANs. Faculty devices are assigned to VLAN 10 and student devices are assigned to VLAN 20. When a broadcast frame is sent from the faculty computer, PC1, to switch S2, the switch forwards that broadcast frame only to those switch ports configured to support VLAN 10.

Figure 3-06

Figure 3-6 Broadcasts with VLAN Segmentation

The ports that comprise the connection between switches S2 and S1 (ports F0/1), and between S1 and S3 (ports F0/3), are trunks and have been configured to support all the VLANs in the network. Port F0/18 is associated with VLAN 20, so S2 forwards the broadcast out port F0/1 but does not forward the broadcast out port F0/18, as shown in Figure 3-6.

When S1 receives the broadcast frame on port F0/1, S1 forwards that broadcast frame out of the only other port configured to support VLAN 10, which is port F0/3. When S3 receives the broadcast frame on port F0/3, it forwards the broadcast frame out of the only other port configured to support VLAN 10, which is port F0/11. The broadcast frame arrives at the only other computer in the network configured in VLAN 10, which is faculty computer PC4.

When VLANs are implemented on a switch, the transmission of unicast, multicast, and broadcast traffic from a host in a particular VLAN is restricted to the devices that are in that VLAN.

Tagging Ethernet Frames for VLAN Identification (

Catalyst 2960 Series switches are Layer 2 devices. They use the Ethernet frame header information to forward packets. They do not have routing tables. The standard Ethernet frame header does not contain information about the VLAN to which the frame belongs. Thus, when Ethernet frames are placed on a trunk, information about the VLANs to which they belong must be added. This process, called tagging, is accomplished by using the IEEE 802.1Q header, specified in the IEEE 802.1Q standard. The 802.1Q header includes a 4-byte tag inserted within the original Ethernet frame header, specifying the VLAN to which the frame belongs.

When the switch receives a frame on a port configured in access mode and assigned a VLAN, the switch inserts a VLAN tag in the frame header, recalculates the FCS, and sends the tagged frame out of a trunk port.

VLAN Tag Field Details

The VLAN tag field, shown in Figure 3-7, consists of a Type field, a Priority field, a Canonical Format Identifier field, and VLAN ID field:

  • Type: A 2-byte value called the tag protocol ID (TPID) value. For Ethernet, it is set to hexadecimal 0x8100.
  • Priority: A 3-bit value that supports level or service implementation.
  • Canonical Format Identifier (CFI): A 1-bit identifier that enables Token Ring frames to be carried across Ethernet links.
  • VLAN ID (VID): A 12-bit VLAN identification number that supports up to 4096 VLAN IDs.

    Figure 3-07

    Figure 3-7 802.1Q VLAN Tag

After the switch inserts the Type and tag control information fields, it recalculates the FCS values and inserts the new FCS into the frame.

Native VLANs and 802.1Q Tagging (

The behavior of frames in the context of IEEE 802.1Q trunking is a vestige of the original standard, which was created when VLANs were still widely used. Essentially, the behavior is dictated by the assumption that a hub is connected between two switch ports that define a common VLAN trunk.

Tagged Frames on the Native VLAN

Some devices that support trunking add a VLAN tag to native VLAN traffic. Control traffic sent on the native VLAN should not be tagged. If an 802.1Q trunk port receives a tagged frame with the VLAN ID the same as the native VLAN, it drops the frame. Consequently, when configuring a switch port on a Cisco switch, configure devices so that they do not send tagged frames on the native VLAN. Devices from other vendors that support tagged frames on the native VLAN include IP phones, servers, routers, and non-Cisco switches.

Untagged Frames on the Native VLAN

When a Cisco switch trunk port receives untagged frames (which are unusual in a well-designed network), it forwards those frames to the native VLAN. If there are no devices associated with the native VLAN (which is not unusual) and there are no other trunk ports (which is not unusual), the frame is dropped. The default native VLAN is VLAN 1. When configuring an 802.1Q trunk port, a default Port VLAN ID (PVID) is assigned the value of the native VLAN ID. All untagged traffic coming into or out of the 802.1Q port is forwarded based on the PVID value. For example, if VLAN 99 is configured as the native VLAN, the PVID is 99 and all untagged traffic is forwarded to VLAN 99. If the native VLAN has not been reconfigured, the PVID value is set to VLAN 1.

In Figure 3-8, PC1 is connected by a hub to an 802.1Q trunk link. PC1 sends untagged traffic, which the switches associate with the native VLAN configured on the trunk ports, and forwards accordingly. Tagged traffic on the trunk received by PC1 is dropped. This scenario reflects poor network design for several reasons: It uses a hub, it has a host connected to a trunk link, and it implies that the switches have access ports assigned to the native VLAN. But it illustrates the motivation for the IEEE 802.1Q specification for native VLANs as a means of handling legacy scenarios.

Figure 3-08

Figure 3-8 Native VLAN Forwarding Behavior

Voice VLAN Tagging (

Recall that to support VoIP, a separate voice VLAN is required.

An access port that is used to connect a Cisco IP Phone can be configured to use two separate VLANs: one VLAN for voice traffic and another VLAN for data traffic from a device attached to the phone. The link between the switch and the IP phone acts as a trunk to carry both voice VLAN traffic and data VLAN traffic.

The Cisco IP Phone contains an integrated three-port 10/100 switch. The ports provide dedicated connections to these devices:

  • Port 1 connects to the switch or other VoIP device.
  • Port 2 is an internal 10/100 interface that carries the IP phone traffic.
  • Port 3 (access port) connects to a PC or other device.

On the switch, the access is configured to send Cisco Discovery Protocol (CDP) packets that instruct an attached IP phone to send voice traffic to the switch in one of three ways, depending on the type of traffic:

  • In a voice VLAN tagged with a Layer 2 class of service (CoS) priority value
  • In an access VLAN tagged with a Layer 2 CoS priority value
  • In an access VLAN, untagged (no Layer 2 CoS priority value)

In Figure 3-9, the student computer PC5 is attached to a Cisco IP Phone, and the phone is attached to switch S3. VLAN 150 is designed to carry voice traffic, while PC5 is in VLAN 20, which is used for student data.

Figure 3-09

Figure 3-9 Voice VLAN Tagging

Sample Configuration

Example 3-2 shows sample output. A discussion of voice Cisco IOS commands is beyond the scope of this course, but the highlighted areas in the sample output show the F0/18 interface configured with a VLAN configured for data (VLAN 20) and a VLAN configured for voice (VLAN 150).

Example 3-2 Default VLAN Configuration

S1# show interfaces f0/18 switchport
Name: Fa0/18
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 20 (student)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: 150 (voice)
<output omitted>
5. VLAN Implementations (3.2) | Next Section Previous Section

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