Ask Dr. Intranet

I am implementing Dynamic Host Configuration Protocol (DHCP) in an intranet with eight subnets. From the router, IP helper points to two DHCP servers. We have the subnet mask, 255.255.254.0, making two virtual subnets for each real subnet. The first DHCP server handles all even numbers of each virtual subnet, and the other handles odd numbers. If one DHCP server fails, the other can give IP addresses for any subnet. The problem is, clients only take IP addresses from one server unless it is out of IP addresses or it fails. Is there a way to sync scopes on the two servers or a method of creating a backup DHCP server?
Jim Paulin, server manager, Ford Motor Co.

The IETF is working on the server-to-server communications specification you'll need to get your DHCP servers to work in such a fashion. Until it finishes the specification, you are trapped by your router's implementation of helper addresses and the client's DHCP implementation. It sounds like the first helper address listed in your router always responds first, and your clients accept the first DHCP offer they receive.

If you'd like to modify your DHCP servers, look at the free WIDE DHCP source code package available at www.sfc.wide.ad.jp/
~tomy/dhcp/index-e.html. Also take a look at the Host/ Subnet Quantities Table listed at www.cisco.com/warp
/public/701/8.html to see if your subnet mask is partitioning your network the way you'd expect.

As a network architect at Change at Work in Houston, Blass understands the strain of developing and managing intranets. Send your problems to dr.internet@changeatwork.com

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