Subject: [rbridge] Trivial (I hope) 802 VLAN encoding question

From: Radia.Perlman at sun.com (Radia Perlman)
Date: Sun, 07 Jan 2007 18:52:50 -0800

Is there a distinction between VLAN 0 and null VLAN? In other words, if 
there is
no VLAN tag, is that the same as being in VLAN 0?

I'm trying to specify how to distinguish the per-VLAN instance of IS-IS 
from the core instance.
I was assuming it would be done based on an inner VLAN tag. But suppose 
there were no VLANs,
but you still wanted different instances for the endnode information and 
the core information (so that
truly inner guys wouldn't have to store all the endnode membership link 
state information).
You couldn't do that with a VLAN tag, unless you used VLAN tag with 
VLAN=0 for the endnoe
membership instance.

Not a really horrible problem -- we can certainly find a way of encoding 
this, but just wondering if
VLAN tag will work (where we use VLAN tag explicitly with VLAN = 0 to 
indicate the endnode
membership inforamtion for the null VLAN?)

Radia

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From: touch at ISI.EDU (Joe Touch)
Date: Mon, 08 Jan 2007 16:11:54 -0800

> no VLAN tag, is that the same as being in VLAN 0?

VLAN 0 is special; it means "priority-tagged".

Null VLAN is different.

-- 
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Joe Touch
Sr. Network Engineer, USAF TSAT Space Segment

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From: Donald.Eastlake at motorola.com (Eastlake III Donald-LDE008)
Date: Tue, 9 Jan 2007 10:30:34 -0500

Well, I may be confused and don't claim to thoroughly understand all
this, but it seems to me there are three cases:

(1) You can have a frame with no Q/VLAN tag at all. Obviously, in some
sense, this has the "null" VLAN.

(2) You can have a frame with a Q/VLAN tag that, in some sense, only
specifies priority. This is indicated by setting the VLAN tag to zero.
According to the table on page 76 of IEEE 802.1Q-2005, there are three
"special" VLAN tag values. That table states that zero is "The null VLAN
ID." and indicates that the tag has only priority information. The other
special values are one, which is the default VLAN ID if not configured
otherwise, and 0xFFF which is "reserved for implementation use".

(3) You can have a frame with a Q/VLAN tag that provides both priority
and VLAN ID. Such a tag would have a VLAN tag in the inclusive range of
0x001 to 0xFFE.

[(4) You can not have a Q/VLAN tag the provides only a VLAN ID. If such
a tag is present, then whatever the value in the three bit priority
field is always specifies a priority for the frame.]

Donald

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
Behalf Of Joe Touch
Sent: Monday, January 08, 2007 7:12 PM
To: Radia Perlman
Cc: rbridge at postel.org
Subject: Re: [rbridge] Trivial (I hope) 802 VLAN encoding question

Radia Perlman wrote:
> Is there a distinction between VLAN 0 and null VLAN? In other words, 
> if there is no VLAN tag, is that the same as being in VLAN 0?

Null VLAN is different.