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Understanding VLAN implementation and IP address assignment

How do you configure VLANs in a domain environment? How will clients get their IPs, and is it possible to communicate across different VLANs in this environment? What will the configuration requirement be on the DHCP server?

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To answer the first question, the answer is always: it depends. Configuring VLANs is fairly straightforward based on the platform chosen in the switching environment to support VLAN administration. Unfortunately the configuration is actually the easiest part, the hard part is designing an adequate VLAN design.

Once you have that, configuration is clearly documented in the switch manual. To get a good VLAN design, it is imperative that you understand the network, application distribution model, and user access methods.

Clients do not get their IP's based on the VLAN assignment model. That is traditionally a question that is handled by either deploying a DHCP server (generally used for client machines) or statically assigning IP addresses (generally used for DNS management of servers). Often times this is more of a political decision (got to love the layer 8 issues) than a technical one; but a fairly good design model is to use DHCP servers for client workstations, set up into DHCP zones and to use DNS and manual IP assignment for servers.

The configuration requirements for DHCP servers traditionally depend on the size of your network and how you logically want to break up these networks. Most notably, I see DHCP zones logically set up by geographic region. This configuration tends to provide better stability over routing and switched environments and, coupled with the proper VLAN configuration, makes localized traffic more local which is always a good thing.

This was first published in March 2008

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