Sams Teach Yourself MCSE Windows NT Server 4 in 14 Days
(Publisher: Macmillan Computer Publishing)
Author(s): David Schaer, et al
ISBN: 0672311283
Publication Date: 12/15/97

This field is used to refer to the client only within DHCP Manager and does not have to match the computer’s actual NetBIOS name. However, using the NetBIOS name simplifies administration.

5.  You also may add any comments about the client in the Client Comment field.

After you have added a reservation, you can view and manage it as described in the previous section.

Configuring Other DHCP Options

Previously, it was mentioned that additional information could be included with the DHCP information during client initialization. The following are some of the more commonly used parameters that can also be passed to the client:

•  003 Router: This is the IP address of a router or default gateway that can be passed along to the client. If a default gateway is already defined on the client, however, it takes precedence over the DHCP information.

•  006 DNS Servers: This is the IP address of any DNS server on the network. Multiple entries are allowed. Remember to enter the address of your primary DNS first.

•  044 WINS/NBNS Servers: Enter here the IP address of any WINS server on the network. You can enter multiple addresses. If any are configured manually on the client, they override the DHCP information.

•  046 WINS/NBT Node Type: This option sets the type of NetBIOS over TCP/IP naming resolution to be used by the client. Valid types are 0x01(b-node or broadcast), 0x02(p-node or peer), 0x04(m-node or mixed), and 0x08(h-node or hybrid).

•  047 NetBIOS Scope ID: Use this option to assign the local NetBIOS scope ID.

Many other options can be configured for a DHCP server to pass along to clients, but the preceding list represents the most commonly assigned ones. These are the only ones that you are likely to use or be tested on.

You can assign to a scope the DHCP scope options just presented in one of three ways:

•  Global Options: These options are passed along to every DHCP client that gets its information from this DHCP server.

•  Scope Options: These options are passed along to every client that leases an address from the particular scope. Scope options override global options.

•  Client Options: These options are those entered manually on the client. Client options override scope options and global options.

To configure scope options, use the following procedure:

1.  From DHCP Manager, choose the scope for which you want to assign scope options.

2.  From the Options menu, select Global or Scope, depending on the level of options you want to assign.

3.  In the Unused Options section, select the option you want to assign. In this case, use 006 DNS Servers, but the procedure is similar for each option. After you select the option, choose Add.

4.  In the Active Options sections, select the option you just added and choose Value to extend the window to edit the values for the options selected. Your window should now look like the one in Figure 11.10.

Figure 11.10.  Editing scope option values.

5.  Choose the Edit Array button and the IP Address Array Editor dialog box in Figure 11.11 should appear.

Figure 11.11.  Editing the IP addresses for a scope option.

6.  Enter the IP address for the DNS server in the New IP Address field and choose Add. You may define multiple DNS servers.

11.2.5. Configuring a BOOTP/DHCP Relay Agent

Any computer running Windows NT 4.0 can be used as an RFC-1542-compliant relay agent. The Microsoft DHCP Relay Agent is an NT networking service and, like other services, can be added by using the Control Panel/Network.

After it is configured, this server can relay DHCP messages, allowing them to be passed to and from computers on different subnets. The server is configured with the IP addresses of known DHCP servers. When a client on the relay agent’s local subnet issues a DHCP Lease Request broadcast, the relay agent intercepts it and forwards it to its list of DHCP servers.

You configure the DHCP relay agent by using the DHCP Relay tab of the TCP/IP Protocol property sheet available from the Control Panel/Network. The configuration is detailed in Figure 11.12.

Under DHCP Servers in the Microsoft TCP/IP Properties dialog box, type the IP address of the server that provides IP addresses to your local subnet and choose Add. You can define multiple servers if necessary. You are now set up as a DHCP relay agent.

Figure 11.12.  Configuring a DHCP relay agent.

11.2.6. DHCP Optimization

Although DHCP does not significantly increase network traffic, you might want to optimize this traffic if network utilization is routinely high. DHCP traffic occurs as the result of one of four events:

•  When a DHCP client initializes for the first time, four broadcast frames occur.

•  When a DHCP client renews its lease, two directed frames are sent. This occurs when the client reboots as well as periodically during the lease period. These renewals occur at 50 percent, 87.5 percent, and 100 percent of the lease period.

•  If a DHCP client replaces its network adapter card, a new request for a new IP address occurs. This involves four broadcast frames.

•  Whenever a DHCP client uses IPCONFIG to renew or refresh its lease, two directed frames are sent. If IPCONFIG is used to release its IP address, then the IPCONFIG renew will result in four broadcast packets.

By lengthening the lease duration, you reduce the frequency of renewal requests, providing the clients do not reboot. The major disadvantage of doing this is that a larger percentage of your available leases will not be available at any given time. If the number of available leases is close to the number of hosts requiring IP addresses, this might result in a lack of availability of needed leases.

The other aspect of DHCP traffic to consider is forwarding of the DHCPDISCOVER messages across BOOTP capable routers. If the router supports configuration of retries before forwarding, increase this number to lengthen the amount of time the host waits for a response before the request is forwarded by the router. This gives the local DHCP server time to respond to the request and keeps this traffic local.