[rbridge] How are packets/link without a VLAN tag handled in a bridged network today?
Michael Smith (michsmit)
michsmit at cisco.com
Wed Jan 4 09:04:08 PST 2006
> -----Original Message-----
> From: rbridge-bounces at postel.org
> --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.
> (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.)
Agreed, normally only 1 VLAN can be sent untagged on a given link.
There are some interesting loop scenarios that occur with 802.1s if you
were to transmit multiple untagged VLANs. However, there are a few
cases where this is done with your multicast example being one.
> BTW... I couldn't find a place in 802.1Q that talks about
> *changing* VLAN IDs; is this a standard function or a "vendor
It was introduced as part of the provider bridging standard, 802.1ad.
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