[j-nsp] RE - 6. vlan-id 0 (Bit Gossip)

Chuck Anderson cra at WPI.EDU
Fri Jul 10 13:05:48 EDT 2009

On Fri, Jul 10, 2009 at 01:23:05PM +0200, sthaug at nethelp.no wrote:
> Is your goal to be able to handle untagged traffic on this link? With
> IQ2 PICs (or GigE ports on an MX) you can handle both untagged, single
> tagged and dual tagged Ethernet frames on the same link. Forget about
> VLAN id 0, the relevant question here is untagged or not.

Actually, there is this rare breed of Ethernet frames that have a tag, 
but specify VLAN ID 0.  These are sometimes called 802.1p tagged 
frames, since the other part of the 802.1Q frame that everyone forgets 
about is the 802.1p [1] bits for link-layer QOS.  You can't set the 
p-bits without using an 802.1Q tagged frame, so if you actually want 
to treat the traffic as belonging to the default port VLAN as assigned 
to the port on the router or switch rather than a specific VLAN 
number, you use VLAN 0 in the frame.

[1] http://en.wikipedia.org/wiki/IEEE_802.1p

I have no idea how JUNOS or Cisco deal with such frames, however.

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