I configured my OpenStack installation and all went Ok. I used the Open vSwitch plugin with GRE tunnels and although I had ping connectivity, when I try to connect to instances through ssh I got a time out. The problem seemed to be in the tunnel MTU size. I had to lower the mtu size on instances to prevent packet fragmentation over GRE tunnel.
Edit /etc/neutron/dhcp_agent.ini file, add this line:
# Override the default dnsmasq settings with this file dnsmasq_config_file = /etc/neutron/dnsmasq/dnsmasq-neutron.conf
Create file /etc/neutron/dnsmasq/dnsmasq-neutron.conf and add these values:
Finally restart neutron server
# service neutron-server restart