VLAN 0 Priority Tagging Support

Last Updated: December 4, 2012

The VLAN 0 Priority Tagging Support feature enables 802.1Q Ethernet frames to be transmitted with the VLAN ID tag set to zero. These frames are called priority tagged frames. Setting the VLAN ID tag to zero allows the VLAN ID tag to be ignored and the Ethernet frame to be processed according to the priority configured in the 802.1P bits of the 802.1Q Ethernet frame header.

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Restrictions for VLAN 0 Priority Tagging Support

  • QinQ is not supported with this feature.

Information About VLAN 0 Priority Tagging Support

802.1Q Tagging

The 802.1Q standard defines a system of VLAN tagging for Ethernet frames and also contains a provision for a quality of service (QoS) prioritization scheme known as 802.1P, which indicates the priority level of the frame. The 802.1Q standard adds this information to the Ethernet header, as shown in the figure below. The priority level values range from zero (best effort) to seven (highest). These values can be used to prioritize different classes of traffic such as voice and video. The VLAN ID tag specifies the VLAN to which the frame belongs. The priority bits define the priority with which the frames are processed.

Figure 1 802.1Q Ethernet Frame


Native VLANs

When a particular VLAN ID is assigned as a native VLAN, frames that are sent from that native VLAN subinterface are not tagged. Similarly, any untagged frames received on that subinterface are associated with the native VLAN. A native VLAN sends only untagged frames, but can receive both tagged and untagged frames.

VLAN 0 Priority Tagging Overview

The VLAN 0 Priority Tagging feature is installed on the customer premises equipment (CPE). In the illustration below, the 802.1Q frames are sent in the upstream direction from the CPE to the internet service provider (ISP). The frames are transmitted with the 802.1Q VLAN tag set to zero and the 802.1P priority bits configured as per the priority with which the frames are to be processed. When these frames are received at the ISP end, the header is stripped off and the frame is processed as per the configuration of the 802.1P priority bits. If the VLAN ID has a nonzero value, the header is retained and the frame is transmitted to the specified VLAN subinterface. High priority frames are sent ahead of low priority frames, and this prioritization is weighted, that is, low priority traffic is not completely suppressed even if high priority traffic exceeds the line rate.

Figure 2 VLAN 0 Priority Tagging


The VLAN 0 Priority Tagging Support feature also allows VLAN 0 to be set as a native VLAN using the encapsulation priority-tagged native command. Setting the VLAN 0 subinterface as a native VLAN allows this subinterface to receive both tagged and untagged frames but transmit only untagged frames. The encapsulation priority-tagged native tx-tagged command configures the native VLAN with VLAN 0 to receive both tagged and untagged frames but to transmit only tagged frames.

How to Configure VLAN 0 Priority Tagging Support

Configuring VLAN 0 Priority Tagging

SUMMARY STEPS

1.    enable

2.   configure terminal

3.   interface type number

4.   encapsulation priority-tagged

5.   end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Device> enable

 
Enters privileged EXEC mode.
  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Device# configure terminal

 
Enters global configuration mode.  
Step 3
interface type number


Example:

Device(config)# interface Ethernet 0/0.1

 
Configures an interface and enters subinterface configuration mode.  
Step 4
encapsulation priority-tagged


Example:

Device(config-subif)# encapsulation priority-tagged

 
Sets the VLAN ID tag of the subinterface to zero.  
Step 5
end


Example:

Device(config-subif)# end

 
Exits subinterface configuration mode and returns to privileged EXEC mode.  

Configuring a VLAN 0 Subinterface as a Native VLAN

SUMMARY STEPS

1.    enable

2.   configure terminal

3.   interface type number

4.   encapsulation priority-tagged

5.   encapsulation priority-tagged native

6.   end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Device> enable

 
Enables privileged EXEC mode.
  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Device# configure terminal

 
Enters global configuration mode.  
Step 3
interface type number


Example:

Device(config)# interface Ethernet 0/0.1

 
Configures an interface and enters subinterface configuration mode.  
Step 4
encapsulation priority-tagged


Example:

Device(config-subif)# encapsulation priority-tagged

 
Sets the VLAN ID tag of the subinterface to zero.  
Step 5
encapsulation priority-tagged native


Example:

Device(config-subif)# encapsulation priority-tagged native

 
Configures the priority-tagged subinterface as a native VLAN.  
Step 6
end


Example:

Device(config-subif)# end

 
Exits subinterface configuration mode and returns to privileged EXEC mode.  

Configuring Native VLAN to Transmit Tagged Frames

SUMMARY STEPS

1.    enable

2.   configure terminal

3.   interface type number

4.   encapsulation priority-tagged

5.   encapsulation priority-tagged native

6.   encapsulation priority-tagged native tx-tagged

7.   end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Device> enable

 
Enters privileged EXEC mode.
  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Device# configure terminal

 
Enters global configuration mode.  
Step 3
interface type number


Example:

Device(config)# interface Ethernet 0/0.1

 
Configures an interface and enters subinterface configuration mode.  
Step 4
encapsulation priority-tagged


Example:

Device(config-subif)# encapsulation priority-tagged

 
Sets the VLAN ID tag of the subinterface to zero.  
Step 5
encapsulation priority-tagged native


Example:

Device(config-subif)# encapsulation priority-tagged native

 
Configures the priority tagged subinterface as a native VLAN.  
Step 6
encapsulation priority-tagged native tx-tagged


Example:

Device(config-subif)# encapsulation priority-tagged native tx-tagged

 
Configures the priority tagged native VLAN interface to transmit tagged frames.  
Step 7
end


Example:

Device(config-subif)# end

 
Exits subinterface configuration mode and returns to privileged EXEC mode.  

Configuration Examples for VLAN 0 Priority Tagging Support

Example: Configuring a Priority Tagged Native VLAN Interface to Transmit Tagged Frames

Device> enable         
Device# configure terminal
Device(config)# interface Ethernet 0/0.1
Device(config-subif)# encapsulation priority tagged
Device(config-subif)# encapsulation priority-tagged native 
Device(config-subif)# encapsulation priority-tagged native tx-tagged
Device(config-subif)# end

Additional References

Related Documents

Related Topic Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

ATM commands

Cisco IOS Asynchronous Transfer Mode Command Reference

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Feature Information for VLAN 0 Priority Tagging Support

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for VLAN 0 Priority Tagging Support
Feature Name Releases Feature Information

VLAN 0 Priority Tagging Support

15.2(3)T

The VLAN 0 Priority Tagging Support feature enables 802.1Q Ethernet frames to be transmitted with the VLAN ID tag set to zero.

The following command was introduced or modified: encapsulation priority-tagged.

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