Vlan0 for 802.1P traffic.

[Aureliano]

The question is from ENCOR learning. Why we need separate vlan (vlan0) for 802.1P traffic (CoS, QoS) if we already have 3-bit field reserved in any 802.1Q packet for QoS? What is actually flowing through vlan0? 

[brselzer · Accepted solution]

Hello,

My understanding is that 802.1p is just 802.1q with the vlan ID set to zero. Someone correct me if I am wrong. If you want to send something out of an access port or native vlan, you don't add an 802.1q tag so you lose the QoS markings. What if I wanted to preserve the QoS markings but didn't need the Vlan ID? You could use 802.1p which keeps the CoS bits but doesn't have any particular vlan assigned so the device interprets it as the "native vlan" whichever one that happens to be.

In reality, how devices actually interact with an 802.1p tag is hit or miss and can cause some strange issues. 

Hope that helps!

[Aureliano]

Thank you for answering. It sounds reasonable. As I understand there is no way to assign any port to vlan0, so how even is it possible for packets to flow in vlan0?

[brselzer]

Hello,

There is no actual Vlan that is "Vlan0". It means if the tag as a value of 0 then treat it as if it were untagged.

Hope that helps!

[Aureliano]

Got it. Thank you. So is vlan 0 is the same as default native vlan (vlan 1), untagged, but with CoS bits?

[brselzer]

Correct

[Joseph W. Doherty]

"As I understand there is no way to assign any port to vlan0, so how even is it possible for packets to flow in vlan0?"

Well, if the device had a way to set CoS without setting the VLAN ID tag too, that should generate a frame that looks like a .1Q frame, but as noted by Bradley, it should be treated as untagged, for VLAN ID.

BTW in https://en.wikipedia.org/wiki/IEEE_802.1Q

VLAN identifier (VID)
A 12-bit field specifying the VLAN to which the frame belongs. The hexadecimal values of 0x000 and 0xFFF are reserved. All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs. The reserved value 0x000 indicates that the frame does not carry a VLAN ID; in this case, the 802.1Q tag specifies only a priority (in PCP and DEI fields) and is referred to as a priority tag. On bridges, VID 0x001 (the default VLAN ID) is often reserved for a network management VLAN; this is vendor-specific. The VID value 0xFFF is reserved for implementation use; it must not be configured or transmitted. 0xFFF can be used to indicate a wildcard match in management operations or filtering database entries

[Aureliano]

Thanks for the explanation. Now it's clear. Good reference about vendor-specific default assignments. I never had a chance to see/manage different-vendor network