Dynamic Host Configuration Protocol

DHCP and Routing and Remote Access

The DHCP service can be deployed along with Routing and Remote Access to dynamically provide remote access clients with IP addresses during a dial-up connection. When Routing and Remote Access and DHCP are used together on the same server computer, the information provided during dynamic configuration is provided differently from that provided under the normal DHCP configuration for LAN-based clients.

When a remote access server provides dynamic configuration for dial-up clients, the remote access server first performs the following steps before the client is assigned DHCP lease information:

When the remote access server starts with the Use DHCP to assign remote TCP/IP addresses option, it makes advance requests to the DHCP server to obtain DHCP client addresses for dial-up clients, and caches these addresses.
The number of client addresses that are requested in advance is equal to the number of Routing and Remote Access ports set to receive calls, plus one additional address.

For example, if the remote access server has two analog modem ports and two ISDN adapter ports that are set to receive calls, the remote access server requests a total of five IP addresses from the DHCP server. The first four are for assigning to remote access clients that dial into the Routing and Remote Access ports. The fifth address is reserved for the remote access server computer, to configure and use as its own IP address when processing connections for dial-up clients.

When the remote access server uses this type of proactive caching of DHCP address leases for dial-up clients, it records the following information for each lease response it obtains from the DHCP server:

The IP address of the DHCP server.
The client's leased IP address (for later distribution to the remote access client).
The time at which the lease was obtained.
The time at which the lease expires.
The length of the lease.

All other DHCP option information returned by the DHCP server (such as server global, scope, or reserved client options) is discarded. When the client dials into the remote access server and requests an IP address (that is, Server Assigned IP Address is selected), the remote access server uses one of these cached leases to provide the remote access client dynamic IP address configuration. When the IP address is provided to the remote access client, the client is unaware that the IP address has been obtained through this intermediate communication between the DHCP server and the remote access server. The remote access server maintains the lease on behalf of the client. Therefore, the only information that the client receives from the DHCP lease is the IP address.