Subject: [rbridge] How are packets/link without a VLAN tag handled in a bridged network today?

From: Eric.Gray at marconi.com (Gray, Eric)
Date: Wed, 4 Jan 2006 12:04:45 -0500

Harald,

	It is not necessary to specify "changing" VLAN IDs, nor
is it precisely a vendor extension.

	It is simply something that can happen inside a box if 
a specific VLAN has one ID on one port and another ID on another
port.

	About the only thing that would need to be said is that 
the VLAN ID and actual VLAN identified by it must be common to
all ports attached to the same shared LAN segment.  In order 
to keep things simple, it would not make sense to have different
VLAN IDs used to identify the same VLAN in a highly connected
set of LAN segments.  But this makes sense for other reasons as 
well, since the most likely motivation for needing to change 
VLAN IDs is because of scoping issues with VLAN IDs assigned by 
multiple authorities and - in such cases - it is unlikely that
this will occur among highly interconnected LAN segments.

	It is probably worth noting that the only time a VLAN 
aware bridge/switch would otherwise typically need to look at 
VLAN IDs is when it needs it to disambiguate filtering database
entries (when the same MAC address appears on multiple VLANs).
This is essentially what makes VLAN aware and non VLAN aware
bridges/switches compatible.

--
Eric 

--> -----Original Message-----
--> From: rbridge-bounces at postel.org 
--> [mailto:rbridge-bounces at postel.org] On Behalf Of Harald 
--> Tveit Alvestrand
--> Sent: Wednesday, January 04, 2006 5:36 AM
--> To: Developing a hybrid router/bridge.
--> Subject: Re: [rbridge] How are packets/link without a VLAN 
--> tag handled in a bridged network today?
--> 
--> 
--> 
--> --On torsdag, desember 29, 2005 20:03:10 -0800 "Ghanwani, Anoop" 
--> <anoop.ghanwani at hp.com> wrote:
--> 
--> > As for (3), yes, a port can be a member of
--> > multiple VLANs.  In that case, untagged traffic
--> > arriving at the port gets assigned the PVID.
--> > When you send traffic out, there is a parameter
--> > to control whether or not a given VLAN should
--> > be tagged/untagged before sending the frame
--> > out.
--> 
--> Just to be sure I understand you correctly:
--> 
--> If an untagged frame arrives from the LAN to the switch, it 
--> gets assigned 
--> the PVID belonging to the port it arrives on (there can be 
--> only one).
--> 
--> The operator can configure the port to receive traffic for 
--> multiple VLANs, 
--> and can make the decision for each VLAN on the port whether 
--> to send out the 
--> frames on that port for that VLAN tagged or untagged.
--> 
--> Right?
--> 
--> (I can imagine some cases where it would be useful to send 
--> traffic from 
--> more than one VLAN untagged (if one VLAN is used to distribute 
--> unidirectional multicast traffic "broadcast-like" into all 
--> segments, for 
--> instance), but I agree that this could easily be confusing 
--> if the operator 
--> doesn't keeep things very straight in his head.... mixing 
--> tagged and 
--> untagged traffic would chiefly be useful (I think) if a 
--> link is used both 
--> to connect end-stations to bridges and to connect bridges, 
--> much as we 
--> imagine "internal" (in Joe's terms) traffic between 
--> RBridges flowing over 
--> the same physical LAN segments as "external" traffic.)
--> 
--> BTW... I couldn't find a place in 802.1Q that talks about 
--> *changing* VLAN 
--> IDs; is this a standard function or a "vendor extension"?
--> 
-->                       Harald
--> 
--> 
--> 
--> 
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