[rbridge] Trivial (I hope) 802 VLAN encoding question
sgai at nuovasystems.com
Tue Jan 9 13:57:05 PST 2007
Inside a bridge ALL frames are VLAN tagged. The cases of NULL VLAN, VLAN
0 or no VLAN do not exist once the frame enters the bridge. These are
only link concepts.
Each port has a Port_VLAN parameter that is assigned by management. On
many switches its default value is 1.
When a port receives the frame it does the following:
if (1Q tag exists)
if (1Q.VID == 0)
VLAN = Port_VLAN'; // Priority tagged frame
VLAN = 1Q.VID; // 1Q tagged frame
VLAN = Port_VLAN; // Untagged frame
Therefore you can assume that any frame inside an RBridge has an inner
This is consistent with IEEE 802.1Q that states:
VLAN technology introduces the following three basic types of frame:
a) Untagged frames;
b) Priority-tagged frames; and
c) VLAN-tagged frames.
An untagged frame or a priority-tagged frame does not carry any
identification of the VLAN to which it belongs. Such frames are
classified as belonging to a particular VLAN based on parameters
the receiving Port, or, through proprietary extensions to IEEE 802.1Q
standard, based on the data content of the frame (e.g., MAC Address,
layer 3 protocol ID, etc.).
A VLAN-tagged frame carries an explicit identification of the VLAN to
which it belongs; i.e., it carries a tag header that carries a non-null
VID. Such a frame is classified as belonging to a particular VLAN based
on the value of the VID that is included in the tag header. The presence
of the tag header carrying a non-null VID means that some other device,
either the originator of the frame or a VLAN-aware Bridge, has mapped
this frame into a VLAN and has inserted the appropriate VID.
> -----Original Message-----
> From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org]
> Behalf Of Radia Perlman
> Sent: Sunday, January 07, 2007 6:53 PM
> To: rbridge at postel.org
> Subject: [rbridge] Trivial (I hope) 802 VLAN encoding question
> Is there a distinction between VLAN 0 and null VLAN? In other words,
> 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
> from the core instance.
> I was assuming it would be done based on an inner VLAN tag. But
> there were no VLANs,
> but you still wanted different instances for the endnode information
> the core information (so that
> truly inner guys wouldn't have to store all the endnode membership
> 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
> 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?)
> rbridge mailing list
> rbridge at postel.org
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