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Community Member

what is vlan0

Hi All,

One of our wireless was setup to send vlan0. I recently found it in the packet capture. I couldn't find any thing that could refer to vlan 0.

Any reference link will also do the help.

Regards,

Ravi

10 REPLIES
Cisco Employee

Re: what is vlan0

Hi Ravi,

The VLAN ID 0 is used when a device needs to send priority-tagged frames but does not know in which particular VLAN it resides. The basic Ethernet frame does not have any priority field. The priority bits, also called CoS bits (Class of Service) are a part of 802.1Q VLAN tag. Therefore, a device needing to add a CoS marking to its frames has to insert a 802.1Q tag into each frame. However, even though this device may be capable of adding 802.1Q tags into its frames, it may not know in what VLAN it currently resides.

This is where the VLAN ID 0 comes in. A device that sends CoS-marked frames can insert a 802.1Q tag into a frame, use the VLAN ID 0 and set the CoS marking appropriately. When a VLAN-aware switch receives this frame, the VLAN ID 0 tells it: "Put the frame in the ordinary access VLAN of the port as if it was untagged, however, process the CoS field accordingly." In other words, the VLAN ID 0 represents the access - or the native - VLAN of the receiving port, whatever VLAN that might be.

Please feel welcome to ask further!

Best regards,

Peter

Community Member

what is vlan0

Hi Peter,

It makes sense now. Can you refer any link where I do further reading.

Regards,

Ravi    

Cisco Employee

what is vlan0

Hi Ravi,

Download the 802.1Q standard from

http://standards.ieee.org/getieee802/download/802.1Q-2005.pdf

Then proceed to the page 76 and see the Section 9.6, Table 9-2.

Best regards,

Peter

Community Member

what is vlan0

Hi Peter,

However, the COS feild here is set to 0. Packet number is 11.

Please check the screenshot.

Regards,

Ravi

Cisco Employee

what is vlan0

Hello Ravi,

No problem. This frame simply has the CoS explicitly set to 0. If it was not marked, the switch could theoretically assign some different default CoS to it. However, here the CoS is specified explicitly - even though it is set to 0, which is the default value for untagged frames anyway. There's nothing wrong with it.

Best regards,

Peter

Community Member

what is vlan0

Hi Peter,

Please tell me if these packets when sent on trunk port will retain the VLAN ID 0. Meanwhile I will go through the document.

Regards,

Ravi

Cisco Employee

what is vlan0

Hi Ravi,

Please tell me if these packets when sent on trunk port will retain the VLAN ID 0.

No, they will not. They will instead be tagged with the access VLAN of the port they were received on.

Best regards,

Peter

Community Member

what is vlan0

Hi Peter,

OK. Perfectly makes sense.

Regards,

Ravi

Community Member

Re: what is vlan0

What is the purpose of VLAN 4095?

Community Member

Re: what is vlan0

this is fine, but the question is we are tagging priority to vlan 0.. but how we come to know which packet is marked with higher priority... this is not cleare which packet is marked....

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