I have been using LDAP Authentication Module (Drupal 6.x) successfully with our network's AD server for a couple of years, no problem. (note: I use an LDAP account with stripped down permissions for non-anonymous search).
So ... our sysadmin people just added a second AD server to our network for redundancy. They wanted me to test to see if Drupal's LDAP would work with both servers. I added the server in the LDAP Module configuration. It passed the authentication Test for non-anonymous search using my LDAP account mentioned above. I can disable either of the servers from the Module settings, and authentication still takes place.
However, if either of the AD servers goes offline, Drupal's LDAP does not find the second server. The only way I can make it work is to go in to the LDAP settings and deactivate the server that went offline. According to documentation on http://drupal.org/node/118123, "the LDAP Servers will be consulted in the order they are listed. They are listed in the order they are created. Future releases will include an option to re-order them for authentication." This rollover doesn't seem to be happening, and I'm not sure where to go from here. Searched the issue queue but did not find this one.
Any enlightenment is welcome.
Gary Broyhill www.lr.edu