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.

I did that but it did not work. please explain it easier. please see and edit my config:

left switch:

interface Ethernet0/0/10
 port link-type access
 port default vlan 100
 loopback-detect packet vlan 100
 loopback-detect enable
 undo ntdp enable
 undo ndp enable
 port-security enable
 port-security protect-action protect     
 port-security max-mac-num 5
 multicast-suppression 5
 broadcast-suppression 10
 traffic-pppoe any d0d0-fd50-7e90

interface GigabitEthernet0/0/2
 port link-type trunk
 undo port trunk allow-pass vlan 1
 port trunk allow-pass vlan 100 1083 to 1084
 loopback-detect packet vlan 100
 loopback-detect enable
#

right switch:

interface GigabitEthernet0/0/2
 port link-type trunk
 port trunk allow-pass vlan 100
 loopback-detect packet vlan 100
 loopback-detect enable

interface Ethernet0/0/1
 port link-type access
 port default vlan 100
 loopback-detect packet vlan 100
 loopback-detect enable