DHCP-SERVER----router----SW--- private line-----USG---
1. It cannot get IP address after configuring DHCP CLIENT ENABLE in Physical sub interface, It can get the IP address normally when PC with VLAN label encapsulation replace USG. It is found that USG sends DHCP DICOVER packets, The other side does not send OFER packets.
TTL of USG is 1, TTL of PC is 64, there is a router among users’ DHCP SERVER and USG, make DHCP RELAY on the router, the router will reduce the value of TTL, that make USG cannot receive DISCOVERY sent by USG after the DHCP SERVER sending.