Ensuring DHCP Server Availability

by Mitch Tulloch [Published on 29 March 2005 / Last Updated on 29 March 2005]

Forget the 80/20 rule for DHCP server availability and use 50/50 instead.

The traditional 80/20 approach to DHCP server availaibility works well enough unless it takes longer than a few hours to get a failed DHCP server up and working again. Or unless you've configured leases shorter than three days because your network is a classroom, a conference facility, or has a lot of mobile users who connect to it remotely. Because of considerations like these, many network administrators prefer a more conservative 50/50 rule, which actually works even better in a single subnet environment anyway, provided both servers have enough addresses scoped to cover the needs of all the clients on the network. Then if one server goes down, the other one can take up the slack for as long as it takes to get the first one up and running again. The typical way of implementing this is to use exclusions, for example:

Server 1
Scope to
Exclusion to

Server 2
Scope to
Exclusion to

But you can also simply configure the scopes separately as:

Server 1 scope to

Server 2 scope to

Just don't let the scopes overlap when you configure them!

See Also

The Author — Mitch Tulloch

Mitch Tulloch is a well-known expert on Windows Server administration and cloud computing technologies. He has published over a thousand articles on information technology topics and has written, contributed to or been series editor for over 50 books.

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