ZyXEL VES1724-56B2 User Guide - Page 313
DHCP Relay
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Chapter 35 DHCP 35.4 DHCP Relay Configure DHCP relay on the Switch if the DHCP clients and the DHCP server are not in the same broadcast domain. During the initial IP address leasing, the Switch helps to relay network information (such as the IP address and subnet mask) between a DHCP client and a DHCP server. Once the DHCP client obtains an IP address and can connect to the network, network information renewal is done between the DHCP client and the DHCP server without the help of the Switch. The Switch can be configured as a global DHCP relay. This means that the Switch forwards all DHCP requests from all domains to the same DHCP server. You can also configure the Switch to relay DHCP information based on the VLAN membership of the DHCP clients. 35.4.1 DHCP Relay Agent Information The Switch can add information about the source of client DHCP requests that it relays to a DHCP server by adding Relay Agent Information. This helps provide authentication about the source of the requests. The DHCP server can then provide an IP address based on this information. Please refer to RFC 3046 for more details. The DHCP Relay Agent Information feature adds an Agent Information field (also known as the Option 82 field) to DHCP requests. The Option 82 field is in the DHCP headers of client DHCP request frames that the Switch relays to a DHCP server. 35.4.2 DHCP Relay Agent Information Format A DHCP Relay Agent Information option has the following format. Figure 195 DHCP Relay Agent Information Option Format Code Length i1 i2 iN (82) (N) ... i1, i2 and iN are DHCP relay agent sub-options, which contain additional information about the DHCP client. You need to define at least one sub-option. 35.4.3 Sub-Option Format There are two types of sub-option: "Agent Circuit ID Sub-option" and "Agent Remote ID Suboption". They have the following formats. Figure 196 DHCP Relay Agent Circuit ID Sub-option Format SubOpt 1 (1 byte) Length N (1 byte) String or System Name Value Figure 197 DHCP Relay Agent Remote ID Sub-option Format SubOpt 2 (1 byte) Length N (1 byte) String or Port Name Value VES1724-56 User's Guide 313