Interaction Between the DHCP Relay Agent, DHCP Client, and DHCP Servers

In a typical carrier edge network configuration, the DHCP client is on the subscriber’s computer, and the DHCP relay agent is configured on the router between the DHCP client and one or more DHCP servers.The following steps describe, at a high level, how the DHCP client, DHCP relay agent, and DHCP server interact in a configuration that includes two DHCP servers.

Note: To prevent DHCP spoofing, you can add to the above hardware configuration an MX-series router. In this configuration, the MX-series router would act as a snooping agent and would be connected to the DHCP relay agent and the client. For more information about preventing DHCP spoofing, see .

1. The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters for the subscriber, including an IP address.
2. The DHCP relay agent receives the discover packet and forwards copies to each of the two DHCP servers. The DHCP relay agent then creates an entry in its internal client table to keep track of the client’s state.
3. In response to receiving the discover packet, each DHCP server sends an offer packet to the client. The DHCP relay agent receives the offer packets and forwards them to the DHCP client.
4. On receipt of the offer packets, the DHCP client selects the DHCP server from which to obtain configuration information. Typically, the client selects the server that offers the longest lease time on the IP address.
5. The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.
6. The DHCP relay agent receives the request packet and forwards copies to each of the two DHCP servers.
7. The DHCP server requested by the client sends an acknowledgement (ACK) packet that contains the client’s configuration parameters.
8. The DHCP relay agent receives the ACK packet and forwards it to the client.
9. The DHCP client receives the ACK packet and stores the configuration information.
10. If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.
11. After establishing the initial lease on the IP address, the DHCP client and the DHCP server use unicast transmission to negotiate lease renewal or release. The DHCP relay agent “snoops” on all of the packets unicast between the client and the server that pass through the router to determine when the lease for this client has expired or been released. This process is referred to as lease shadowing or passive snooping.

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