Open Stack

I'm sorry to disturb you. 
Recently, I tried to install openstack through devstack. When I input "./stack.sh". I can install openstack successfully. 
Then I tried to create a cloud instance and use the public network 172.24.4.0/24 which is created during installation( this subnet is created by default, I didn't configure network informartion in local.conf before installation). And the instance can access to the Internet smoothly.
But the instance will not access the Internet when I reboot my server  (physical machine). After rebooting, I input "sudo ifconfig br-ex 172.24.4.1/24 up", the instance can access my server IP, but it can't PING the gateway addresses of my server. Of course, the instance also can't access the Internet. But my server can PING it's gateway and access to the Internet. Finally, the cloud instance can only communicate with my server.
I tried many methods to restore the network environment of my openstack. But I can't find the reason. So I need your help. I install the version of devstack is stable/train. Thank you very much!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack-discuss/attachments/20200822/c4032a3b/attachment-0001.html>