Zhone zNID-GPON-2402 Configuration Manual page 201

VLANS
value is 8100. This family of products allows that field to be specified by the
user.
Figure 159: S-Tagged on uplink, tagged LAN
On the web interface, the S-tag feature is defined on the VLAN mode page as
shown below. Once enabled, all VLAN traffic being sent upstream will have
the outer S-tag applied to the packet.
Figure 160: Stag is set from the VLAN Service Mode dropdown
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zNID 24xx Series Configuration Guide