DVS-108 Series Managed Industrial Ethernet Switches User Manual DHCP Relay Statistics 3.1.5.3 DHCP L2Relay In some networks, DHCP servers rely on Relay Agent Information option appended by Relay Agents for IP address and other parameter assignment policies. This works fine when end hosts are directly connected to Relay Agents. In some network configurations, one or more Layer 2 devices may reside between DHCP clients and a Relay agent. In these network scenarios, it is difficult to use the Relay Agent Information option for an IP address and other parameter assignment policies effectively.
So there is a requirement for the device that is closest to the end hosts to append a Relay Agent Information option in DHCP messages. These devices are typically known as Layer 2 Relay Agents. DHCP snooping steps: 1. A DHCP client sends a DHCP request via broadcast. 2. When a switch (relay agent) receives the DHCP request, it will add DHCP option-82 to the packet. DHCP option-82 includes the MAC address of the host which sends a DHCP request (remote-ID sub-option) and the interface ID on the switch which connects to the host (circuit-ID sub-option). 3. If the switch has configured an IP address, the IP address will be added into the DHCP packet. 4.
If a DHCP server supports option-82, after the DHCP server receives the DHCP request, it will 3-18 Item Description No of Packets inserted Circuit-Id option The amount of Packets which inserted Circuit-Id option. No of Packets inserted Remote-Id suboption The amount of Packets which inserted Remote-Id suboption. No of Packets dropped The amount of Packets which dropped. No of Packets which did not inserted RAI option The amount of Packets which did not insert RAI (Relay Agent Information) option.