I have always though that ssh-agent has some limitations. One of those limitations is that when invoked from .bashrc or .zshrc in the following way:

`eval ssh-agent`

will cause one ssh-agent instance to be spawned for every shell, which is a waste of resources. An easy solution is to use Keychain, which is also described here.
Basically, Keychain [...]

Reusing existing OpenSSH v4 connections comments a very interesting feature of OpenSSH 4: reusing open connections.

ControlMaster
Enables the sharing of multiple sessions over a single network connection. When set to “yes” ssh will listen for connections on a control socket specified using the ControlPath argument.
ControlPath
Specify the path to the control socket used for connection sharing as [...]