Actual Problem
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while a list of LDAP servers is possible there isn't a built-in timeout mechanism in Keystone to failover to the next LDAP server in the list if there is no response.  Try setting your first LDAP server in the list to a server which will not respond on 636 i.e. behind a firewall that silently drops packets.  What you will find is Keystone will hang waiting for a connection timeout and keystone authentication will timeout.
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Replicated the issue and here is the result
++++++++++++++++++++++++++++++++++++++++++++++

My keystone auth config for the domain /etc/keystone/domains/keystone.LAB.conf

~~~~~~~~~~~
[ldap]
url =  ldaps://ipb.test.com,ldaps://ipa.test.com
user = uid=svc-ldap,cn=users,cn=accounts,dc=test,dc=com
user_filter = (memberOf=cn=grp-openstack,cn=groups,cn=accounts,dc=test,dc=com)
password = redhat
user_tree_dn = cn=users,cn=accounts,dc=test,dc=com
~~~~~~~~~~~

Both of the ldap server are IPA

When it works and goes to ldaps://ipa.test.com

- When we stop IPA service on ipb.test.com
- When we shutdown the ldap/ldaps port on ipb.test.com

When it do not work

- Drop the packet like # ipatables -I INPUT -s OSP-Controller -j DROP

- Network stop responding 

** But its work well when it " Destination Host Unreachable" (Manually delete the arp from the table)

=======================================

The configuration of ldap redundancy been configured through 

https://bugzilla.redhat.com/show_bug.cgi?id=1298769