There's a lot of information on the intertoobs about getting ssh-agent "working" in OS X and even more articles about when and how the stock behavior of ssh-agent changed (mostly with respect to how ssh-agent interacted with the Keychain).
This article doesn't cover or care about any of that.
This article is concerned with:
- Enabling ssh-agent in such a way that I can "ssh-add" in one terminal window and that same agent (and the loaded keys) is available in all of my other terminal windows.
- Enabling use of ssh-agent from MacPorts and/or Homebrew and not the older ssh-agent that OS X ships with in /usr/bin.
- To avoid having to put my keys in the Keychain (just a matter of preference).