Operation Manual – DHCPH3C S7500 Series Ethernet Switches Chapter 4 DHCP Snooping Configuration4-3To do… Use the command… RemarksEnter system view system-view —Enable the DHCPsnooping function dhcp-snoopingRequiredBy default, the DHCPsnooping function isdisabledEnter Ethernet port view interface interface-typeinterface-number —Set the port connected toa DHCP server to atrusted portdhcp-snooping trustRequiredBy default, all ports of aswitch are untrusted portsNote:z DHCP relay agent and DHCP snooping cannot be enabled at the same time. If youhave enabled DHCP relay agent on the device, you will fail to enable DHCPsnooping.z The dhcp-snooping trust command and the dhcp-snooping command must beconfigured at the same time; otherwise DHCP packets may be dropped in actualnetworking.4.2 DHCP-Snooping Option 824.2.1 Overview of DHCP-Snooping Option 82I. Introduction to DHCP Option 82For details of option 82, see Option 82 Supporting.II. Working mechanism of DHCP-Snooping option 82The process in which a DHCP client obtains an IP addresses from a DHCP serverthrough DHCP-Snooping is the same as the process in which a DHCP client obtains anIP address from a DHCP server directly. The process includes four phases: IP leaserequest, IP lease offer, IP lease selection, and IP lease acknowledgement. This sectiononly introduces the working mechanism of DHCP-Snooping option 82. The details areas follows:1) When a DHCP client gets online, it broadcasts an IP address request messageacross the network.2) After receiving the broadcast message, the DHCP-Snooping-enabled switchchecks whether the message contains option 82 and processes it.
