RRAS and DHCP Leasing

Are your DHCP scopes doomed to be drained?

At a recent conference at which I spoke, a session attendee asked me about the following RRAS and DHCP scenario: When you select the Windows 2000 Routing and Remote Access Server Setup Wizard's Virtual private network (VPN) server configuration option (which Figure 1 shows), RRAS configures a large number of available VPN connections (i.e., 128 PPTP connections and 128 Layer 2 Tunneling Protocol—L2TP—connections) for incoming clients. I had problems with Windows NT RAS and RRAS exhausting my DHCP scopes when obtaining DHCP leases for dial-in RAS clients. Based on the number of VPN connections that Win2K RRAS configures, this situation will go from bad with NT to worse with Win2K. Are my DHCP scopes doomed to be drained?

I'm happy to report good news: Win2K RRAS handles DHCP leasing more intelligently than NT 4.0 RAS and RRAS do. Win2K's DHCP leasing behavior is different from the NT 4.0 method, in which the NT RAS server leases enough IP addresses for all the RAS devices configured on the RAS server (e.g., modems, ISDN terminal adapters, PPTP VPN adapters), as well as one address for the RAS server interface. For example, if you've configured 15 modems for use with your NT 4.0 RAS or RRAS server, the server leases 16 IP addresses when RAS or RRAS starts up.

In Win2K, RRAS uses the more intelligent method of leasing IP addresses from a DHCP server in pools or groups on an as-needed basis. This process prevents RRAS from wasting valuable IP addresses.

If a DHCP server is available and you've configured a Win2K RRAS server to use that DHCP server for client addressing, the RRAS server leases IP addresses in blocks of 10 and stores the addresses in the Registry to allocate to clients later. If the RRAS server's address pool is exhausted, the server leases additional addresses from the DHCP server in blocks of 10. The efficiency of this block-by-block method prevents the DHCP lease-draining problems that are common on NT 4.0 RAS and RRAS servers that support many clients. The key benefit of this leasing method is that you can increase or reduce the size of the IP address pool allotments as you see fit.

Depending on your situation, you might determine that you need to modify the default number of DHCP leases that Win2K RRAS obtains from the DHCP server. You can change the number of addresses that RRAS leases at one time by editing the HKEY_LOCAL_ MACHINESYSTEMCurrentControlSetServices RemoteAccessParametersIP Registry subkey. The default value of the InitialAddressPoolSize subkey (of type REG_DWORD) is 10.

This Win2K RRAS and DHCP question was probably on the minds of many administrators who have already used PPTP or L2TP to set up RRAS with VPN connections. These administrators, as well as the conference attendee who asked the question, can breathe a sigh of relief to know that Win2K RRAS servers handle DHCP server interaction and client IP address allocation more intelligently than their NT 4.0 predecessors do.