Managing Host Routes Using DHCP Relay Proxy

The DHCP relay proxy feature enables the E Series router to efficiently manage host routes for DHCP clients, including:

• Installing routes when DHCP clients are configured
• Removing routes when DHCP clients release their DHCP-assigned addresses or when the addresses expire

When a DHCP client sends a request to an external DHCP server, the relay proxy receives the request and forwards it to the external DHCP server. The relay proxy then sends the DHCP server’s response back to the client. This process is similar to that used by the DHCP relay component. The DHCP client views the relay proxy as a DHCP server, and the DHCP server sees the relay proxy as a DHCP relay agent.

To DHCP clients, there is no difference when they use a DHCP relay or a DHCP relay proxy. However, the DHCP relay proxy differs from the DHCP relay in how client address renewals and releases are handled:

• With the DHCP relay proxy, DHCP clients communicate with the relay proxy to renew and release addresses.
• With the DHCP relay, DHCP clients communicate directly with the DHCP server to renew and release addresses.

A major benefit of the relay proxy configuration is that the E Series router is kept informed of the status of a DHCP client’s address. When addresses are released by clients, the router removes the installed host route for that client. In the DHCP relay configuration, the router does not know when addresses have been renewed or released; the host routes that are no longer needed are still unavailable.

For additional information on managing client bindings, see Viewing and Deleting DHCP Client Bindings.

Selecting the DHCP Server Response

Similar to the DHCP relay, the DHCP relay proxy enables you to specify up to five DHCP servers to provide address and configuration information for a DHCP client. As an added benefit over the relay, when using multiple DHCP external servers, you can configure how the DHCP relay proxy determines which offer to send to the DHCP client. You can configure the DHCP relay proxy to use either the single best offer or the first offer it receives from the DHCP servers.

If there are multiple offers, the DHCP relay proxy selects the final offer based on the following priorities:

1. The offer that contains the IP address requested by the DHCP client.
2. The offer that contains an IP address on the same subnetwork as the requested IP address.
3. The offer that has the longest lease time.

If you have enabled the optional select-first-offer feature, the DHCP relay proxy immediately uses the first offer that it receives from any DHCP server.

Behavior for Bound Clients and Address Renewals

When a DHCP client is already bound to an IP address or is renewing the lease on its IP address, DHCP relay proxy unicasts DHCP ACK and DHCP NAK replies to the client regardless of the current configuration of the set dhcp relay layer2-unicast-replies command or the set dhcp relay broadcast-flag-replies command. These commands control the transmission method used for DHCP reply packets.

This behavior applies only to DHCP relay proxy; it does not apply to DHCP relay because DHCP relay does not maintain a list of active clients or receive address renewal requests from clients.

For information about using the set dhcp relay layer2-unicast-replies command, see Configuring the Layer 2 Unicast Transmission Method for Reply Packets to DHCP Clients. For information about using the set dhcp relay broadcast-flag-replies command, see Configuring the Layer 2 Unicast Transmission Method for Reply Packets to DHCP Clients.