Both sides of the device have been configured LLDP, but the VSM cannot found the link automatically
Configure LLDP way:
Globally enable LLDP, all interface opens LLDP function.
Interfaces enable LLDP, then this interface opens LLDP function.
This client globally enabled LLDP, all the interfaces enable LLDP function.
Enable LLDP on the middle device or cancel the device, the problem is resolved.
Because the client network environment is complex, client start the LLDP overall on all layer 2 switch, but finds one single switch, which is not connected to other switches. Since the VSM server can control this switch, it means that switch has a link connected with gateway, but LLDP cannot find this link.
According to checkup, it is found that there is another layer 2 switch between two devices, and this device is not controlled by VSM, did not enable LLDP which led to this link cannot be found.
It will find the link only when LLDP be configured on the direct link, layer 2 switch cannot put between it, but hub can be put.