Question: I am a developer who is used to being able to test and debug code. 

Occasionally I have to make changes to our load balancer configuration. As far as I can see, if I mess this up it could stop the whole site working, but we don't have a way to test it off-line.

How do people test such things? I was hoping there would be some sort of emulator that I could use. Or is it possible to have a second configuration for testing?

I would like to be as confident in the changes I make to the load balancer as I am to the changes I make to my code. Does anyone have a test suite they use for testing their load balancer?

**update**

We are in the process of moving from one back end to another, and we are redirecting users based on the url.

Answer: 
Other load balancers offer similar functionality to F5, and their vendors provide more useful test and development options.

You can get a development license for Zeus Traffic Manager (full functionality) or a Citrix VPX license (standard edition only), both limited to 1 MBits throughput (which should be adequate for most development purposes), and valid for 1 year.

Zeus will renew development licenses yearly at no charge (don't know what Citrix will do?)

Aaron - under no circumstances is a test virtual server on a production unit a good alternative!
I do this by setting up a second virtual (or set of virtuals) that are mapped to the same pools. You can then test the staging VIP by editing your /etc/hosts file (or Windows hosts file).

At this point I'll sing the praises of my outsourced DNS provider, Dynect (<http://dynect.com>). They have a Traffic Management feature that allows you to add both the old and new VIPs to a hostname rotation, but also adjust the weighting - the ratio of how often the old vs new IP is served, and then you can raise the weight to slowly migrate if a hard cutover is too risky.

For internal DNS, I believe that F5's GLB can do the same thing (but don't quote me as I don't have hands-on experience here).