NH=> Excellent question. I have made exactly this same
observation in the past. In principle there is no reason why you cannot
re-use all the codepoints of each/every field in the traffic unit because, as
stated, although the traffic units look identical they do belong to quite
different layer networks. I think there are 2 reasons for
this:
- Firstly, most IEEE folks are not familiar with, and
therefore do not use, functional architecture techniques to describe layer
networks. So this issue may not be so obvious to many folks in
IEEE. Further, when we have QinQ techniques these are a form of
sublayering (like we find in nested LSPs in MPLS) and so we don't have
distinct layer networks here......and although folks may not describe it as
such they do recognise that one of the consequences of this (ie still a single
layer network) is that it affects scaling. However, once we have the
.1ah stuff and MACinMAC (ie true client/server) then everyone intuitively
recognises we have quite disjoint layer networks. So what I am saying
here is that for folks who don't have a strong background in functional
architecture, the observation you have made may not be so obvious even though
it is architecturally true.
- Secondly, there could be
practical/implementation reasons for doing this (ie creating a split in VID
codepoints) when dealing with COTS Ethernet hardware. Maybe some of the
Ethernet hardware experts can provide further clarification
here.