Physical security considerations
Even though I am talking about the network infrastructure, I can't go on without talking about physical security testing that needs to be done. Think about it, you could have all the firewalls, IPS units, VLAN segmentation you need, but if someone can walk right into the datacenter, turn off the server, un-rack it, and leave, those devices won't help at all. This is why the major security standards out there such as PCI require physical security testing as well.
Physical security testing isn't as exciting as attacking the network infrastructure, but is just as important in protecting your infrastructure. The following are some of the major physical security points you will want to test.
Secure access
Secure access is the foundation of physical security. Anywhere there is any sort of space that is not considered common space, there should be a mechanism to restrict access to that space based on the job role. This mechanism can be fob or badge reader of some sort...