Revision 1.3a
Page 49
XLR 550 Manual
4.7
Double Tagging ID Commands
Double tagging is a way to isolate one IEEE 802.1Q VLAN from other IEEE 802.1Q VLANs in a
hierarchical fashion that is compatible with IEEE 802.1Q aware switches as long as those switches
support a maximum frame size of 1526 bytes or more. This method places an extra or Double Tag in
front of a frame
’
s normal tag (assuming the frame was already tagged), increasing the frame size by 4
bytes. The double tag frame format is shown in Figure 7-1.
[Figure 7-1]
If double tagging is enabled on a port, this port removes the first IEEE 802.3ac tag that appears after
the source address in every ingress frame. In this mode, if the ingress frame has a single tag, it is
removed and if the ingress frame has two tags, the first tag is removed.
And this port transmits all egress frames with an extra tag. If a frame is untagged, it egress tagged. If
a frame is tagged, it egress double tagged. The extra or double tag is inserted right after the frame
’
s
source address so this new tag becomes the frame
’
s first tag.
When a tag is added because the port is in double tagging mode, the tag is inserted right after the
egress frame
’
s source address. The four bytes of added data are:
The Tag bits are double tag ID to configure using
“
set doubletag-id
”
command.
The PRI bits indicate the frame
’
s priority determined during ingress.
The CFI bit is always set to a zero.
The VID bits are the frame
’
s source port
’
s PVID and Fixed-Priority.
This section describes the commands that configure and display information about double tagging ID.
To enable double tagging operation, see
“
set port
”
.
* This feature is only supported to XLR550 series.