The Watson Ethernet tabletop modems are transparent to VLANs, i.e. VLAN
tags are preserved during transmission over the DSL.
The Watson Ethernet tabletop modem has a single address learning data-
base which does not take VIDs into account.
3.2.7 Stacked VLANs
The Watson Ethernet plug-in also supports double-tagged VLANs (a.k.a stacked
VLANs). Stacked VLANs is a method to increase the number of available VIDs in
a structured and hierarchical fashion that is backward-compatible with single-
tagged VLANs as long as the network supports packet sizes of 1'526 bytes or
more.
This method places an extra tag (often referred to as the S-TAG or Service Pro-
vider Tag) in front of the first tag (known as the C-TAG or Customer Tag). The
double tag format is shown in Figure 3-11.
Figure 3-11: Stacked VLAN frame format
Each port can be configured for double tagging in incoming (ingress) and out-
going (egress) direction. Normally ingress and egress double tagging are
enabled simultaneously.
A port with ingress double tagging enabled expects double tagged frames. The
port will remove the outer tag. If a frame contains only a single tag then this tag
will be removed. Untagged frames are not modified.
After removal of the outer tag the Ethernet bridge will process the packet as a
single-tagged VLAN packet, cf. 3.2.3. For packets with a single tag or no tag the
default tag for this port will be used to determine VLAN membership and possible
egress ports.
A port with egress double tagging enabled will transmit all packets with an extra
outer tag. When the packet had no tag, it will be sent with a single tag. When the
packet had a single tag, it will egress double-tagged. The tag added is the de-
fault tag of the source port of the packet.
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