VLAN 0 reserved for for 802.1P traffic?

[Vigneshtl]

Hi all,

I have started preparing for CCNP ENCOR 350-401. I have a reference PDF book for CCNP and CCIE Enterprise Core (ENCOR 350-401). In it, I found that VLAN 0 is reserved for 802.1P traffic.

Is this true?

[balaji.bandi]

yes you are correct.

you can find information here :

https://www.cisco.com/c/en/us/td/docs/switches/connectedgrid/cg-switch-sw-master/software/configuration/guide/vlan0/b_vlan_0.html#task_1055372

[Joseph W. Doherty]

Nice reference but want to call mention how reference names specific devices that support using VLAN ID 0 for this purpose.

I.e. such support might not be found on all devices.

Also, IMO, L3 ToS should always be used, whenever possible, over L2 CoS.

BTW, I've never seen a production network using VLAN ID 0.

[Joseph W. Doherty · Accepted solution]

It's possibly better to say VLAN ID 0 is used just to support 802.1P.  Otherwise, some may assume VLAN 0 is just like other VLANs or (more unlikely) only 802.1P can be used with VLAN 0.

I mention the foregoing as, I suspect, we have a diverse readership with regard to networking knowledge.

[MHM Cisco World]

you can use VLAN-ID=0 with voice 
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst1000/software/releases/15_2_7_e/configuration_guides/vlan/b_1527e_vlan_c1000_cg/configuring_voice_vlan.pdf

[Joseph W. Doherty]

Interesting!

Requires a specific option (of which I was unaware of, until now) when defining a voice VLAN on an access port, and without this option configured all such (VLAN ID 0) tagged frames are dropped!  (I wonder how long this option has been supported.)

Mentions accepting, but doesn't mention transmitting, such frames, when configured.(?)

Again, IMO, much better to ignore using L2 CoS if L3 ToS can be used.  However, this does make for wonderful professional or expert level test questions.  ; )

[Vigneshtl]

VLAN refers to the process of segregating a single broadcast domain into multiple broadcast domains. QoS does not require a specific broadcast domain, so why do we need to use VLAN 0?

[Joseph W. Doherty]

". . . so why do we need to use VLAN 0?"

So that we can differentiate between VLAN tag headers used JUST for CoS priority settings, and those used for VLAN membership identification, which can also still use CoS priority settings.

Consider a PC host sending/receiving different classes of traffic (e.g. FTP vs. HTTP) and wants to prioritize these traffic classes differently.  Would you force these different classes to use an explicit/real VLAN ID?  You could, but you don't need to.

Let's look at the beginning of the description for the voice option dot1p:

switchport voice {vlan{vlan-id | dot1p | none | untagged}}

• dot1p: Configures the switch to accept voice and data IEEE 802.1p priority frames tagged with VLAN ID 0 (the native VLAN).

Again, I believe it unlikely you'll see anyone using VLAN ID 0, because there's L3 ToS.

Now, one might argue, if you have just "pure" L2 switches, and if you need to use QoS, you "need" L2 CoS.  Yea, but, my counter argument is, if you have "L2" switches that support QoS and VLANs and whatever (like many Enterprise grade "L2" switches), they are probably intelligent/enhanced "L2" switches that are unable to route (and that's not so true anymore either), but do support L3 packet analysis, for things like ACLs and/or QoS, etc.

Although this voice dot1q option is interesting, the referenced documentation doesn't explain, does the switch generate these kinds of frames, or when accepting them, what happens to the frame going out another port, such as a trunk port.

Personally, my experience has been, when you start using options that are very seldom used, you become the gamma tester, which is part of the "cost of doing business" if you really, really have need of such a rarely used feature, but if there's another way to accomplish the same end goal, and it's more widely used, might the latter be a better choice?

[sidshas03]

No, VLAN 0 is not reserved for 802.1P traffic. In fact, VLAN 0 is not a valid VLAN ID and cannot be used for normal network traffic.

According to the IEEE 802.1Q standard, VLAN IDs range from 1 to 4094. VLAN 0 is used for priority tagging (802.1p) and is often referred to as the priority VLAN. However, it is not a normal VLAN and cannot be used to carry regular data traffic.

In Cisco IOS, VLAN 0 is used to represent the default VLAN. This VLAN is used to assign untagged traffic to a specific VLAN when it arrives on a trunk port. However, this is a Cisco-specific implementation and does not apply to other vendors or the IEEE standard.

So, in summary, VLAN 0 is not reserved for 802.1P traffic and cannot be used for normal network traffic. It is used for priority tagging and is not a valid VLAN ID.

[Joseph W. Doherty]

Some of your statements, at least to me, seem to be contradictory.  Further much, I believe, is factually incorrect.  Of course, I might be mistaken or totally misunderstand what you've written.

For example: "VLAN 0 is used for priority tagging . . ." and "In fact, VLAN 0 is not a valid VLAN ID and cannot be used for normal network traffic." and "However, it is not a normal VLAN and cannot be used to carry regular data traffic."  So, what does VLAN 0 actually priority tag?

Or: "In Cisco IOS, VLAN 0 is used to represent the default VLAN. This VLAN is used to assign untagged traffic to a specific VLAN when it arrives on a trunk port."

I thought, on Cisco switches, the default VLAN was number 1.  I also thought untagged traffic, arriving on a trunk port, was considered part of VLAN 1 unless the trunk port has been configured with "switchport trunk native vlan #".  This is incorrect?  If so, doing a quick Internet search seems many sites have this misinformation too.

Doing some quick searching, on VLAN 0 usage, also came across this Cisco IE2000 configuration guide on VLAN 0 Priority Tagging Support.

This referenced document, appears to answer questions posted in my prior reply to @Vigneshtl.  If I'm reading this document, correctly, seems that purpose of using VLAN ID 0 is to prioritize tag whatever frames you want, without there being any VLAN membership involved.

Can you provide either clarification of what you wrote and/or cite any references making (what would appear to be) the same assertions?

Thanks.

[MHM Cisco World]

For All 
I visit Japan many times, what I love in Japan is using draw/image instead of words, 

so instead write and write you can simple use image, the mind is design for remember image better than words and that why Japanese use image alot 
802.1p and vlan-id =0 is used in Voice to give QoS to frame receive from both PC and Phone, 

[Ryan Brindle]

I personally didn't find the definition in my textbook to be satisfactory and honestly most of the replies in this post are confusing. Now that I have a better understanding of this, I believe I've come up with a concise statement that may help others understand what VLAN 0 is for.

VLAN 0 allows the use of 802.1Q header information in a frame that will be treated as untagged regarding forwarding. This includes two important things one being the PCP priority code point and the DEI drop eligible indicator.

So you can for example throw voice on that VLAN and give it a priority code point which is a three-bit field that indicates class of service which is a part of layer 2 quality of service. And indicate whether or not that frame is drop eligible with the DEI.

I hope this helps clear it up for anybody else who comes across this.