If an interface has been added to VLANs in untagged mode, the interface sends one copy of untagged detection packet regardless of how many VLANs the interface has been added to. The untagged detection packets will be discarded on the link, so loopback detection cannot be performed on the network. In this situation, loopback detection must be configured in these VLANs.
If an interface has been added to VLANs in tagged mode, loopback detection must be enabled in specified VLANs, and the interface sends the loopback detection packets of the VLAN that contains the interface and has loopback detection enabled.
After VLAN IDs are specified, the interface sends one copy of untagged loopback detection packets and multiple copies of tagged loopback detection packets with the specified VLAN tags.