[Samba] How to keep samba4 dc work fine when windows dc offline?

[Sense Zeng]

I'm testing a samba4 server join AD (had 1 win2003 dc) as dc. Version:
Samba4 DC: samba 4.1.1 with ubuntu (172.16.3.8)
Win2003 DC: Win2003 SP2 (172.16.3.3)
AD domain name: dotest.com

The samba server had joined to AD as DC. When win2003 dc online, it seems
works fine.
When I test win2003 dc online, simulate the win2003 DC was failue, found
the samba4 DC can't work fine along. The AD client logon has been very
slow. And there are error log in the samba4 dc log file
/usr/local/samba/var/log.samba:

...
[2013/11/18 09:55:57.147627,  0]
../source4/librpc/rpc/dcerpc_sock.c:256(continue_socket_connect)
  Failed to connect host 172.16.3.3 on port 135 - NT_STATUS_HOST_UNREACHABLE
[2013/11/18 09:55:57.148011,  0]
../source4/librpc/rpc/dcerpc_sock.c:419(continue_ip_open_socket)
  Failed to connect host 172.16.3.3 (880d3cbe-93a7-4e9d-a286-62b54a73a6ea._
msdcs.dotest.com) on port 135 - NT_STATUS_HOST_UNREACHABLE.
[2013/11/18 09:56:00.147628,  0]
../source4/librpc/rpc/dcerpc_sock.c:256(continue_socket_connect)
  Failed to connect host 172.16.3.3 on port 135 - NT_STATUS_HOST_UNREACHABLE
[2013/11/18 09:56:00.148012,  0]
../source4/librpc/rpc/dcerpc_sock.c:419(continue_ip_open_socket)
  Failed to connect host 172.16.3.3 (880d3cbe-93a7-4e9d-a286-62b54a73a6ea._
msdcs.dotest.com) on port 135 - NT_STATUS_HOST_UNREACHABLE.
...

172.16.3.3 is the Win2003 DC

and my smb.conf setting is:

[global]
        workgroup = DOTEST
        realm = dotest.com
        netbios name = GREEN
        server role = active directory domain controller
        server services = s3fs, rpc, nbt, wrepl, ldap, cldap, kdc, drepl,
winbind, ntp_signd, kcc, dnsupdate
        wins support = Yes
[netlogon]
        path = /usr/local/samba/var/locks/sysvol/dotest.com/scripts
        read only = No
[sysvol]
        path = /usr/local/samba/var/locks/sysvol
        read only = No

I wanna keep the samba4 dc work fine when win2003 dc offline? What should I
do?