At the last minute, when 802.1BA was written, we noticed that VLAN 1, by
default, is transmitted without a C-tag. This is a problem, because AVB
streams (pretty much) require a priority field. The fix we came up with,
at the time, was to create the SR_PVID (2), and default AVB traffic to
VLAN 2. This is a problem, because it means that AVB traffic can’t use
VLAN 1 plug-and-play, which interferes with the normal use of VLANs in a
network, e.g. to separate IP subnets.
In retrospect, there was a better solution available. That solution WOULD
HAV BEEN as follows (It’s too late, now, but we need to understand it,
first.):
We have a per-Bridge Port variable that contains two values for “transmit
the PVID on this port tagged/untagged”, one to use if the port is an SRP
core port, and one if it is not. Then, as the port transitions in or out
of core port status, the state of the port can change with respect to
transmitting the default VLAN with or without a tag. The 802.1BA default
would be “untagged, tagged” for “not core, core” respectively. We would
then have no beed for the SR_PVID.
But, since the SR_PVID is there, I would recommend we define that same
variable (in Clauses 12 and 17), but define it for the SR_PVID, not the
PVID, and mention that, if the SR_PVID is configured to equal the PVID,
that variable applies, just the same. Thus, if the Bridge is configured
with SR_PVID = PVID = 1, “core tagged, non-core untagged”, then it works
like I think we should have defined it in the first place. If the Bridge
is configured with SR_PVID = 2, PVID = 1, “tagged, tagged”, it works like
it does, today. Either way, the out-of-the-box default behavior is “PVID
1 is untagged”, as has always been the case.
Of course, this has to be explained in Clause 35.