# DHCP External Server Overview

You can configure the E Series router to provide support for an external DHCP server. This enables the router, which is not running DHCP relay or DHCP proxy server, to monitor DHCP packets and to keep information for subscribers based on their IP address and MAC address. When the E Series router’s DHCP external server application is used, all DHCP traffic to and from the DHCP server is monitored by the router.

The services provided by integrating the E Series router’s DHCP external server application with SRC software are similar to those provided when the DHCP local server is integrated with SRC software. The router’s DHCP external application is used together with other features of the router to provide subscriber management. For additional information, see Configuring Subscriber Management.

| Note: To ensure that DHCP external server with DHCP relay proxy processes unicast reply packets (such as renewal ACK and NAK packets), you must configure DHCP external server with the IP address of the DHCP relay proxy’s giaddr. This configuration ensures that DHCP external server processes renewal ACK packets, which in turn enables the updating of client leases.

Figure 14 shows a network that includes an external DHCP server and the E Series router.

Figure 14: DHCP External Server

In Figure 14, the subscriber requests an address from the DHCP server through the E Series router. All communication between the subscriber and the DHCP server is monitored by the E Series router. After the subscriber receives an IP address, the subscriber is able to access the Internet and use the value-added services provided by the E Series router and by the SAE software. For this to occur, the edge network must be using a DHCP relay function.

When the subscriber requests an IP address from the DHCP server, the E Series router performs the following actions:

- Identifies the subscriber’s IP address, MAC address, giaddr, and client identifier
- Extracts the lease time, creates a shadow lease, and starts its own lease timer that is associated with the subscriber

The E Series router views the subscriber as active once the subscriber sends a packet. The router then performs the following actions:

- Processes the subscriber’s IP address by using a route map
- Extracts the dynamic subscriber interface profile (optional)
- Creates the subscriber’s dynamic subscriber interface

If the SRC software is configured, the router also performs the following actions:

- Alerts the SRC software that the dynamic subscriber interface exists
- Alerts the SRC software that the subscriber’s address exists and provides DHCP options

The SRC software then provides its enhanced services to the subscriber.

The E Series router monitors all traffic between the subscriber and the DHCP server, and resets the shadow lease by monitoring the DHCP server/client lease renewal. When the subscriber disconnects, the shadow lease will eventually expire. The E Series router then performs the following actions:

- Deletes the subscriber’s dynamic subscriber interface
- Alerts the SRC software that the dynamic subscriber interface has been deleted
- Alerts the SRC software that the subscriber’s address has been deleted

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