The VMAN double tag feature adds an optional port CVID parameter to the existing untagged VMAN port configuration. When present, any untagged packet received on the port will be double tagged with the configured port CVID and SVID associated with the VMAN. Packets received with a single CVID on the same port will still have the SVID added. As double tagged packets are received from tagged VMAN ports and forwarded to untagged VMAN ports, the SVID associated with the VMAN is stripped. Additionally, the CVID associated with the configured Port CVID is also stripped in the same operation.
Much like the CVIDs configured as part of the CEP feature, the configured Port CVID is not represented by a VLAN within ExtremeXOS. The implication is that protocols and individual services cannot be applied to the Port CVID alone. Protocols and services are instead applied to the VMAN and/or port as the VMAN represents the true layer-2 broadcast domain. Much like regular untagged VMAN ports, MAC FDB learning occurs on the VMAN, so duplicate MAC addresses received on multiple CVIDs that are mapped to the same VMAN can be problematic. Even when the additional Port CVID is configured, the port still has all of the attributes of a regular untagged VMAN port. This means that any single c-tagged packets received on the same port will have just the SVID associated with the VMAN added to the packet. Likewise, any egress packet with a CVID other than the configured Port CVID will have the SVID stripped.