The figure below shows the processing that occurs on the two relay agents and the DHCP server when this feature is configured:
The DHCP client generates a DHCP message (including option 60) and broadcasts it on the network.
The first DHCP relay agent intercepts the broadcast DHCP request packet and inserts its own option 82 in the packet.
The relay agent automatically adds the circuit ID suboption and the remote ID suboption to option 82 and forwards them to the second relay agent.
The second relay agent encapsulates the first relay agent’s option 82 and inserts its own option 82.
The gateway IP address (giaddr) is set to the incoming interface on the second relay agent and the original giaddr from the first relay agent is encapsulated.
The second DHCP relay agent unicasts the DHCP packet to the DHCP server.
The DHCP server receives the packet and uses the *** suboption information from the second relay agent, along with the option 82 information from the first relay agent, to assign IP addresses and other configuration parameters and forwards the packet back to the second relay agent.
When the second relay agent receives the reply message from the server, it restores the encapsulated option 82 and prior giaddr from the first relay agent. The reply message is then sent to the prior giaddr.
The first relay agent strips option 82 off from the packet before forwarding the packet to the client.
Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent Forwarding
You only need to configure helper addresses on the interface where the UDP broadcasts that you want to forward to the DHCP server are being received. You only need to configure the ip dhcp smart-relay command if you have secondary addresses on that interface and you want the router to step through each IP network when forwarding DHCP requests. If smart relay agent forwarding is not configured, all requests are forwarded using the primary IP address on the interface.
If the ip dhcp smart-relay command is configured, the relay agent counts the number of times that the client retries sending a request to the DHCP server when there is no DHCPOFFER message from the DHCP server. After three retries, the relay agent sets the gateway address to the secondary address. If the DHCP server still does not respond after three more retries, then the next secondary address is used as the gateway address.
This functionality is useful when the DHCP server cannot be configured to use secondary pools.