Why samba on RHEL6 cannot switch backend from 1st ldap server to 2nd one if 1st server has no response?

Issue

Samba cannot switch backend from 1st ldap server to 2nd one if 1st server has no response.

1) Configure backend of samba as follows:

/etc/samba/smb.conf
------------------------------------------------------------------------------
passdb backend = ldapsam:"ldap://<1st ldap server> ldap://<2nd ldap server>"
------------------------------------------------------------------------------

2) Start 1st ldap server.

3) Start 2nd ldap server.

4) Send SIGSTOP signal to 1st ldap server.

 # kill -SIGSTOP <pid>

5) Start smbd.