4.8 DHCPv4

DHCP Snooping Configuration

• Snooping Mode: Indicates the DHCP snooping mode operation. Possible modes are:
  • Enabled: Enable DHCP snooping mode operation. When DHCP snooping mode operation is enabled, the DHCP request messages are forwarded to trusted ports and only allow reply to packets from trusted ports.
  • Disabled: Disable DHCP snooping mode operation
• Port Mode Configuration: Indicates the DHCP snooping port mode. The following are the possible port modes:
  • Trusted: Configures the port as trusted source of the DHCP messages
  • Untrusted: Configures the port as untrusted source of the DHCP messages

DHCPv4 Relay Configuration

A DHCP relay agent is used to forward and to transfer DHCP messages between the clients and the server when they are not in the same subnet domain. It stores the incoming interface IP address in the GIADDR field of the DHCP packet. The DHCP server can use the value of GIADDR field to determine the assigned subnet. For such condition, please make sure the switch configuration of VLAN interface IP address and PVID (Port VLAN ID) correctly. The following figure shows the DHCP Relay Configuration settings.

The list of parameters is as follows:

• Relay Mode: Indicates the DHCP relay mode operation. Possible modes are:
  • Enabled: Enables DHCP relay mode operation. When DHCP relay mode operation is enabled, the agent forwards and transfers DHCP messages between the clients and the server when they are not in the same subnet domain. And the DHCP broadcast message won't be flooded for security considerations.
  • Disabled: Disables DHCP relay mode operation.
• Relay Server: Indicates the DHCP relay server IP address.
• Relay Information Mode: Indicates the DHCP relay information mode option operation. The option 82 circuit ID format as "[vlan_id][module_id][port_no]". The first four characters represent the VLAN ID, the 5^th and 6^th characters are the module ID (in standalone device it always equal 0, in stackable device it means switch ID), and the last two characters are the port number. For example, "00030108" means the DHCP message receive form VLAN ID 3, switch ID 1, port No 8. And the option 82 remote ID value is equal the switch MAC address. Possible modes are: Enabled: Enable DHCP relay information mode operation. When DHCP relay information mode operation is enabled, the agent inserts specific information (option 82) into a DHCP message when forwarding to DHCP server and removes it from a DHCP message when transferring to DHCP client. It only works when DHCP relay operation mode is enabled. Disabled: Disable DHCP relay information mode operation.
• Relay Information Policy: Indicates the DHCP relay information option policy. When DHCP relay information mode operation is enabled, if the agent receives a DHCP message that already contains relay agent information it will enforce the policy. The 'Replace' policy is invalid when relay information mode is disabled. Possible policies are:
  • Replace: Replace the original relay information when a DHCP message that already contains it is received.
  • Keep: Keep the original relay information when a DHCP message that already contains it is received.
  • Drop: Drop the package when a DHCP message that already contains relay information is received.