User guide

Q-in-Q DVLAN (Double-VLAN) Tagging
(802.1AD)
Engineering VLANs
172
pmp-0229 (Mar 2013)
SM4, bottom left in Figure 54, has an existing 802.1ad Q-in-Q network below it, meaning that
packets coming into the SM’s Ethernet interface are already doubly tagged 802.1ad frames. These
frames have an S-Tag VID of 60 and a C-Tag VID of 50. The SM is configured with a Q-in-Q
VLAN Port Type with a Provider VID of 40. If the Provider VID were the same as the underlying
network S-Tag VID (60), then it would not work because the SM would be stripping the outer tag
on the return path. Therefore, in this case, it is important that the SM’s Provider V
ID does NOT
match the S-Tag VID of the Q-in-Q network underneath. In this case, the 802.1ad frames are
passed through untouched above and below the SM.
For the BH units, they must be configured as a Q-in-Q VLAN Port Type if they are to add and
remove S-Tags. In this case, both sides are configured as Q-in-Q Port Types with the BHM having
a Provider VID of 70 and the BHS having a Provider VID of 1. The packet flows shown in Figure
54
show that all but one of them are already 802.1ad Q-in-Q frames when they reach the BHM in
the upstream direction. There is one stream that is a single 802.1Q tagged packet. For this stream,
the BHM will add an S-Tag to it with Provider VID of 70, turning it into an 802.1ad frame. Since
the BHS has Provider VID of 1, it will not add or remove any tag layers. As is the case with regular
VLANs, VID of 1 is a special case in the Canopy network. At this point, all of the packet streams
are Q-in-Q frames going int
o the ISP network. On the return path, Figure 54 shows where the tags
are removed. One stream will have its S-Tag removed by the BHM in the downstream direction
since its S-Tag VID matches the BHM Provider VID. Other streams will have their tags removed
by other devices:
• SM1 will remove both S-Tag and C-Tag leaving an untagged packet for PC1 and PC2.
• SM2 will remove the Q-Tag (its S-Tag was removed by the BHM) from the stream, leaving an
untagged packet for its PC.
• SM3 will remove the S-Tag, leaving the 802.1Q tagged frame for the 802.1Q switch handle
for its PC.
• SM4 will not remove any tags, as it will be sending the 802.1ad frames into the Q-in-Q
network underneath.
In the Figure 54 example above, the S-Tag EtherType is the default 0x88a8 which is specified by
the 802.1ad specification. In order to accommodate interoperability, you may configure a different
EtherType for Q-in-Q packets. This can only be configured on the AP for Multipoint systems, or
the Backhaul Master for point-to-point systems. Once configured on these devices, it will be passed
down to the SMs, or Backhaul Slave, so that both sides of the link will be using the same
EtherType.