Configuring the DHCP Relay Agent Option (Option 82) at Layer 3
After configuring and enabling the DHCP/BOOTP relay feature, you can enable the DHCP relay agent option feature. This feature inserts a piece of information, called option 82, into any DHCP request packet that is to be relayed by the switch. Similarly, if a DHCP reply received by the switch contains a valid relay agent option, the option is stripped from the packet before it is relayed to the client.
When DHCP option 82 is enabled, two types of packets need to be handled:
- DHCP Request
- When the switch (relay agent) receives a DHCP request, option 82 is added at the end of the packet. If the option has already been enabled, then the action taken depends on the configured policy (drop packet, keep existing option 82 value, or replace the existing option). If the incoming DHCP request is tagged, then that VLAN ID is added to the circuit ID sub option of option 82; otherwise, the default VLAN ID is added.
- DHCP Reply
- When the option 82 information check is enabled, the packets received from the DHCP server are checked for option 82 information. If the remote ID sub-option is the switch's MAC address, the packet is sent to the client; if not, the packet is dropped. If the check is not enabled. The packets are forwarded as-is.
The DHCP relay agent option consists of two pieces of data, called sub-options. The first is the agent circuit ID sub-option, and the second is the agent remote ID sub-option. When the DHCP relay agent option is enabled on switches running ExtremeXOS, the value of these sub-options is set as follows:
- Agent circuit ID sub-option
- The full circuit ID string uses the format <vlan_info>-<port_info>. You can use the default values for vlan_info and port_info, or you can configure these values as described later in this section.
- Agent remote ID sub-option
- Always contains the Ethernet MAC address of the relaying switch. You can display the Ethernet MAC address of the switch by issuing the show switch command. You can configure string up to 32 characters as agent remote ID option.
For more general information about the DHCP relay agent information option, refer to RFC 3046.