To DHCP, or not to DHCP: That is the Question.

My apologies in advance (or belated in this case) to William Shakespeare. This morning, I met with a customer and had a rather in-depth whiteboard session with two of their senior architects. We were discussing how to use Collage in a production deployment. I usually start with our standard technical presentation (gotta love PowerPoint), but I quickly find myself drawn to the whiteboard (or "grease board" as one of our Sales reps is fond of saying).

All technical environments (with IT as no exception) have their own systems of best practices, dogmas and religious beliefs. DHCP versus fixed IP addressing for servers falls somewhere in between dogma and religion. However, as you start moving towards utility computing, your data center can take on a more dynamic persona. For example, applications (and servers) could be provisioned as they are needed to respond to increasing workloads. Servers can also be re-purposed during the day, as your data center takes on different application profiles. A given server could be an e-mail server in the morning, a web server in the afternoon and a business intelligence server at night.

Cassatt Collage allows you to manage more static, traditional data centers and also allows you to manage a more dynamic, utility-computing environment. In order to repurpose a server, Collage takes advantage of DHCP. A server's IP address is assigned by Collage's DHCP service. However, Collage allows you to control precisely what IP addresses are allocated to your applications.

Let's take the following example of a typical three-tier application:

• Web tier: up to 20 servers with IP addresses 10.20.120.40 - 10.20.120.59
• App server tier: up to 10 servers with IP addresses 10.20.120.60 - 10.20.120.69
• Database tier: up to 5 servers with IP addresses 10.20.120.70 - 120.20.120.74

When you create these tiers in Collage, you can specify the IP addresses available to each tier. In this manner, you have precise control over your IP address space and how different applications map to your network topology. The Collage Network Virtualization Service (NVS) also allows you to specify a VLAN or network segment for each application tier.

When servers are allocated to an application tier at run time, each server is given an IP address by the Collage DHCP service. Even though these servers take advantage of dynamic IP addressing, their IP address can be constrained ahead of time. If you are taking advantage of NVS, Collage will can create a new VLAN for you and will automatically program the layer-2 switches in your data center. This allows you precise control of how servers are mapped into your environment. A particular server's application stack and network identification, however, are determined dynamically when that server is allocated to an application tier.