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DHCP relay is a Layer 3 feature. When the DHCP client and DHCP server are not in the same physical subnet, DHCP relay can be deployed on the Layer 3 device laid between the DHCP client and DHCP server to forward DHCP messages, implementing allocation of IP addresses to the network hosts across different Layer 3 subnets.
DHCP Snooping and DHCP Relay Networking Applications
As shown in figure 1, an enterprise has Department A, Department B and Department C, the gateway server of the enterprise network serves as the DHCP server. The hosts in the three departments are not in the same subnet with the DHCP server. The enterprise wants to dynamically assign IP addresses to the hosts of all the departments by the same DHCP server. The network administrator can deploy a DHCP relay agent between the hosts and the DHCP server to achieve this.
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Option 82 contains two common sub-options: Circuit ID and Remote ID. Circuit ID is mainly used to identify the VLAN and the interface of the client, Remote ID identifies the device from which the client accessed, usually the MAC address of the device.
Packet Format
Circuit ID Sub-option
Circuit ID includes three types: Type 0 (value is port index), Type 1 (value is port name + VLAN ID) and Type 2 (value is port description + VLAN ID). The format is listed below by types.
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DHCP snooping Option 82 can be enabled on Switch 1, and DHCP relay Option 82 can be enabled on Switch 2. It is optional. But if Option 82 is inserted on Switch 1, DHCP relay Option 82 trust all should be enabled on Switch 2.
Option 12 and Option 42
DHCP options are additional parameters that can be passed from a DHCP server to DHCP clients during the IP address assignment process. These options provide additional configuration information beyond just the IP address and subnet mask. In addition to supporting Option 82, PICOS also supports Option 12 and Option 42.
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Option 12: This option is used to specify the hostname of the DHCP client. When a DHCP client requests an IP address from a DHCP server, it can include its hostname in the DHCP request message using Option 12. The DHCP server can then use this information to assign a hostname to the client along with its IP address lease.
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Interaction with Other Protocols
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