This article would not be complete without a look at measures a LAN administrator can take to mitigate the effects of DHCP outages. There are some basic rules that you can apply when designing or upgrading a DHCP Server installation. Not only will they keep productivity high, but they'll also make you look good when you report how easily you solved a problem. 80/20
Why put all your eggs in one basket? In the context of DHCP servers, the 80/20 rule refers to the common practice of splitting DHCP scope ranges into 80 percent and 20 percent portions, which are managed by different physical DHCP servers. That way, if one of the servers dies, clients can still get IP address information from the other machine, greatly reducing the impact of DHCP server outages. Big companies with very large scopes and superscopes often use this method. However, it can also be effective in medium-size and smaller firms. Rogues
Unless you use Windows 2000 DHCP servers, your company could be a victim of rogue DHCP servers. This is a situation in which a new DHCP server comes on line and starts servicing client IP requests. This can be, and often is, benign; for example, someone testing some functionality may have forgotten to disconnect the test machine from the LAN. The potential for confusion is great -- and occasionally comical -- in such situations. Microsoft addressed the issue in Windows 2000 by making it compulsory to authorize DHCP servers in Active Directory, which only Enterprise Administrators can do. Routers
If routers segment a LAN, then unless there's a DHCP server on each segment, client DHCP Discover packets will have to cross the routers to find DHCP servers. If the router is BOOTP capable, this is a nonissue. However, if the router is older and not BOOTP capable, the subnet must have a DHCP Relay Agent installed to capture the DHCP Discover packets and forward them to the DHCP server. This is fine, but it adds another layer of complexity and management, thereby increasing both the chances of failure and the LAN administrator's workload. Fault tolerance
It goes without saying that whenever possible, the elements of a corporate network should be duplicated through fault tolerance. In DHCP terms, this means installing at least two DHCP servers, each of which is capable of stepping into the breach if required. You should also mirror physical DHCP fault tolerance in DHCP software by using the 80/20 rule for further protection. Conflict Detection
Once you have a working Windows 2000 DHCP installation, you can turn on an extremely useful feature known as Conflict Detection. This feature will ping your LAN for a specific IP address before it adds it to the pool of available IP addresses the DHCP server will lease. You can turn it on by opening the DHCP console and selecting Properties from the Action menu. When the properties window appears, click on the Advanced tab. Under Conflict Detection Attempts, enter a number greater than 0. This number sets the number of times a ping is sent to the LAN. You should use this feature sparingly because it ups the load on the server by increasing the DHCP server's response time to client requests. Microsoft recommends that you set Conflict Detection Attempts value no higher than 2. Turn Conflict Detection off once you're satisfied that your DHCP database is in good shape again.
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