Open Stack

> When i was trying to use DHCP in openstack i found openstack DHCP
> start provide ip address to my existing LAN machines ( we are using
> flat VLAN with neutron), that is why i disable openstack DHCP, Is it
> common or i am doing something wrong?

I do not think this should happen.
It has been a while (Folsom) since I touched a setup with mixed “LAN” and OpenStack DHCP but IIRC it works like this:
    
AFAIK the leases file neutron uses is very specific and will only reply to the mac-addresses that are in the dnsmasq config.
Looking at the dnsmasq process it is set to static:
From the man page:
The optional <mode> keyword may be static which tells dnsmasq to enable DHCP for the network specified, but not to dynamically allocate IP addresses, only hosts which have static addresses given via dhcp-host or from /etc/ethers will be served.

Usually the problem is the other way around:
The existing DHCP in the “lan” bites with what OpenStack does. (so an OpenStack instance gets an IP from the lan DHCP)
This can be prevented by blacklisting the MAC address range your instances get in your lan dhcp (Blacklist MAC starting with fa:16:3e )

Cheers,
Robert van Leeuwen