Configuring DHCP Proxy Clients

DHCP proxy client support enables the router to obtain an IP address from a DHCP server for a remote PPP client. Each virtual router (acting as a DHCP proxy client) can query up to five DHCP servers.

For PPP users, the router acts as a DHCP client to obtain an address for the user. This is referred to as DHCP proxy.

The process for PPP users is as follows:

1. The remote user dials in, and the client requests RADIUS authentication.
2. The AAA server on the router sends a request to the DHCP proxy client on the router for an IP address to be assigned to the remote user’s host.
3. The proxy client assumes the role of DHCP client and sends a discovery message to each DHCP server.
4. One or more of the DHCP servers responds with an offer message containing an IP address.
5. The proxy client determines which offer to accept and sends a message to that DHCP server requesting that IP address.
6. The DHCP server responds to the proxy client with an acknowledgment message.
7. The proxy client passes the IP address to the authentication, authorization, and accounting (AAA) server on the router, and the AAA server returns the address to PPP. PPP then assigns the address to the remote host. The new IP address is included when the router next updates its routing table.

Dynamic IP addresses are leased to the remote host for a specific period of time, which can range from minutes to days. At the halfway point in the lease period, the proxy client requests an extension from the DHCP server on behalf of the remote host. The lease is extended for a period specified in the acknowledgment (ACK) message returned by the DHCP server—typically equal to the original lease. If the DHCP server returns a negative acknowledgment (NAK) message to the proxy client, the proxy client notifies the server on the router that the extension has been denied. The AAA server logs out the remote host and frees the IP address for reuse.

When a remote host disconnects, the AAA server notifies the proxy client that the IP address is available for reuse. The proxy client informs the DHCP server, which can now reassign that IP address.

Note: The maximum number of DHCP proxy client bindings that are stored on the router chassis is 48,000.

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

To configure a proxy client from Global Configuration mode:

1. Specify the address of the DHCP server that will provide IP addresses for remote hosts. You can specify a maximum of five DHCP servers.
   host1(config)#ip dhcp-server 10.6.128.10
2. Direct the router to request IP addresses for remote users from the DHCP server(s).
   host1(config)#ip address-pool dhcp