Due to the fact that the F5 Big-IP make use of SNAT to load balance traffic, your back-end node will see the traffic coming from the IP of the load balancer and not the true client.
To overcome this (for web traffic at least), the F5 injects the X-Forwarded-For header in to HTTP steams with the true clients IP.

In apache you may want to log this IP instead of the remote host if it has been set. Using the SetEnvIf, we can produce a suitable LogFormat line based on if the X-Forwarded-For header is set or not:

CustomLog "/path/o/log/dir/example.com_access.log" combined env=!forwarded
LogFormat "%{X-Forwarded-For}i %l %u %t \"%r\" %s %b \"%{Referer}i\" \"%{User-Agent}i\"" proxy
SetEnvIf X-Forwarded-For "^.*\..*\..*\..*" forwarded
CustomLog "/path/to/log/dir/example.com_access.log" proxy env=forwarded

The above assumes that the “combined” LogFormat has already been defined.

If you use the ::apache::vhost puppet class from the puppetlabs/apache module, you can achieve the same result with the following parameters:

::apache::vhost { 'example.com':
  logroot => "/path/to/log/dir/",
  access_log_env_var => "!forwarded",
  custom_fragment => "LogFormat \"%{X-Forwarded-For}i %l %u %t \\\"%r\\\" %s %b \\\"%{Referer}i\\\" \\\"%{User-Agent}i\\\"\"    proxy
  SetEnvIf X-Forwarded-For \"^.*\..*\..*\..*\" forwarded
  CustomLog \"/path/to/log/dir/${title}_access.log\" proxy env=forwarded"
}