Cannot add new 2008r2 DC into existing domain tree & forest

For testing & studying purpose, I've created 4 virtualbox machines 2 windows 2008 r2, 1 windows 7 Pro, 1 windows XP Pro SP2. One of the 2008r2 has been promo to (root)DC and both win7 and win XP are able to join to the domain. All guest machines are set 2 network adapters, 1 for NAT and another for Bridge.   My objective are adding second dc into existing domain, create a child domain joining the existing domain tree and create another domain tree joining the existing forest.  Here is the TCP/IP configuration of all:

Winows 2008r2 (root DC) [Domain & Forest functional level: 2008]
IP address: 10.0.3.1
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Win 7 Pro
IP: 10.0.3.62
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Win XP Pro
IP: 10.0.3.61
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

2nd Windows 2008r2 (Fresh installed without any role)
IP: 10.0.3.2
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Problem/Question:
I've done only once success to create a new 2008r2 server as a child domain of the root domain.  When I try again,  no matter tried to dcpromo the 2nd 2008r2 to another domain controller or as a child domain, there is always comes out the ERROR_TIMEOUT. Try to ping from both win7 and winXP to 10.0.3.2 and vice verse is success without any problem. However, cannot ping between 10.0.3.2 to root dc(10.0.3.1). I've been tried to disable all the NAT adapter inside the network and only left the "Bridge" having the same issue. Using the nslookup in root dc to check the dns is success.   Anyone can help me? Thanks a lot.