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VLANs in a Multiswitched Environment (3.1.2)

Even a small business might have more than one switch. Multiple switch configuration and design influences network performance. Trunks are commonly used to connect a switch to a switch or to another network device such as a router.

VLAN Trunks (

A VLAN trunk, or trunk, is a point-to-point link between two network devices that carries more than one VLAN. A VLAN trunk extends VLANs across two or more network devices. 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 which 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 3-6, 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.

Controlling Broadcast Domains with VLANs (

Recall that a broadcast domain includes all of the devices that receive a broadcast. When a switch is bought, removed from the packaging, and powered on, all devices attached to the switch are part of the same network or broadcast domain. When VLANs are implemented, each VLAN is its own broadcast domain. Let’s examine that concept because VLANs are commonly implemented in business.

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 the animation, 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.

Network with VLANs

As shown in the animation, 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.

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.

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 that 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.

Figure 3-7 shows a network design without using segmentation compared to how it looks with VLAN segmentation, as shown in Figure 3-8. Notice how the network with the VLAN segmentation design has different network numbers for the two VLANs. Also notice how a trunk must be used to carry multiple VLANs across a single link. By implementing a trunk, any future VLAN or any PC related to assembly line production can be carried between the two switches.

Figure 3-07

Figure 3-7 Network without Segmentation

Figure 3-08

Figure 3-8 Networks with Segmentation

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

Tagging Ethernet Frames for VLAN Identification (

Layer 2 devices use the Ethernet frame header information to forward packets. 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, as shown in Figure 3-9.

Figure 3-09

Figure 3-9 Fields in an Ethernet 802.1Q Frame

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 consists of a Type field, a tag control information field, and the FCS field:

  • Type: A 2-byte value called the tag protocol ID (TPID) value. For Ethernet, it is set to hexadecimal 0x8100.
  • User 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.

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 (

Native VLANs frequently baffle students. Keep in mind that all trunks have a native VLAN whether you configure it or not. It is best if you control the VLAN ID used as the native VLAN on a trunk. You will learn why in this section.

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), the switch 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, then the frame is dropped. The default native VLAN is VLAN 1 on a Cisco switch. 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 in 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 3-10, 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 forward 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. A better designed network without a hub is shown in Figure 3-11.

Figure 3-10

Figure 3-10 Native VLAN on 802.1Q Trunk

Figure 3-11

Figure 3-11 Better Native VLAN Design

Voice VLAN Tagging (

As shown in Figure 3-12, the F0/18 port on S3 is configured to be in voice mode so that voice frames will be tagged with VLAN 150. Data frames coming through the Cisco IP phone from PC5 are left untagged. Data frames destined for PC5 coming from port F0/18 are tagged with VLAN 20 on the way to the phone. The phone strips the VLAN tag before the data is forwarded to PC5.

Figure 3-12

Figure 3-12 Voice VLAN Tagging

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.

When the switch port has been configured with a voice VLAN, the link between the switch and the IP phone acts as a trunk to carry both the tagged voice traffic and untagged data traffic. Communication between the switch and IP phone is facilitated by the Cisco Discovery Protocol (CDP).

Sample Configuration

Look at the sample output.

S1# show interfaces fa0/18 switchport
Name: Fa0/18
Switchport: Enabled
Administrative Mode: static access
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 20 (student)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: 150 (voice)
<output omitted>

A discussion of voice Cisco IOS commands are 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).

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