Network designers make decisions regarding operational complexity every day. Most don't call it that, though; they tend to think along the lines of the difficulty and burden that specific technology places on administrators or users. This section gets to a key aspect of your network security system: achieving operational simplicity can mean the difference between a security system that works for you and a security system that you work for.
Some hard and soft metrics to measure your system include the following:
Don't take the notion of operational simplicity too far, however. For example, operational simplicity is often not improved by topological simplicity. I've heard designers associate topological simplicity with such terms as "elegance." Unfortunately, as any fashion designer will tell you, elegance often flies right in the face of utility, which is a critical aspect of any security system. If you can't respond to the threats you encounter in an easy and obvious way, any amount of "elegance" you achieve through topological simplicity is meaningless.
As an example, let's look at the traditional design of Internet edges. I've seen many network edges that resemble the network shown in Figure 1-4.
Figure 1-4. Traditional Internet Edge Design
Often the network edge looks this way not because it was the best way to design the network, but because the company's security policy dictated that all traffic flow through a single "choke point." Although the design is topologically simple, it has lots of problems. The one on which I focus in this discussion is the potential for human error, or operational complexity. Human error is one of the biggest root causes of configuration problems, especially at 2 a.m. when security administrators find themselves troubleshooting an issue.
Although the design shown in Figure 1-4 might seem simple, in fact it is needlessly difficult because of its operational complexity. The configuration on the firewall and attached switch in this example are so complex and prone to operator error that the slightest mistake causes the entire security deployment to be compromised.
To remedy this problem, create a network that at first glance introduces much more topological complexity, as shown in Figure 1-5.
Figure 1-5. Design with Operational Simplicity
The topology in Figure 1-5 shows an implementation that is easy to understand and much harder to misconfigure. Although it has more devices and doesn't look as "elegant" as the previous example in Figure 1-4, the paths of communication and the insecure and secure parts of the network are much more apparent and securely segmented. In this amended design, outbound Internet access is handled by one firewall, and inbound virtual private network (VPN) access is handled by another firewall with a VPN device. In addition, separate L2 switches are deployed as opposed to relying on a single switch with multiple VLANs. In the heat of the moment, the access rules and potential configuration changes are much simpler.
To ensure operational simplicity, you must constantly evaluate the level of complexity you are comfortable with to ensure that your security is both simple to deploy and straightforward to maintain. Throughout this book, I offer many alternate designs for secure network architectures. Although often some level of topological complexity is introduced to solve a particularly hard problem, all the designs strive toward simplicity of planning, design, implementation, and, most important, operation.