1、review the configuration of usg2210 and switch, and find out whether dhcp assigned ip address and dns automaticly or not.
2、tell users to connect computer with device from our company, the PC could get address and dns normally, without arp collision. The location of problem: internal network problem.
3、review log file of firewall, arp attack exist, message of attack came from internal PC(ip:10.1.2.251).
antivirus is the suggestion, problem solved after antivirus.