[openstack-dev] [Neutron][Nova][DHCP] Use case of enable_dhcp option when creating a network

John Kasperski jckasper at linux.vnet.ibm.com
Wed Dec 17 15:53:45 UTC 2014

When enable_dhcp is False, Config drive or metadata service can be used 
to assign static IP addresses to the deployed VM if the image has 
cloud_init or something equivalent.

On 12/17/2014 2:15 AM, Padmanabhan Krishnan wrote:
> Hello,
> I have a question regarding the enable_dhcp option when creating a 
> network.
> When a VM is attached to  a network where enable_dhcp is False, I 
> understand that the DHCP namespace is not created for the network and 
> the VM does not get any IP address after it boots up and sends a DHCP 
> Discover.
> But, I also see that the Neutron port is filled with the fixed IP 
> value from the network pool even though there's no DHCP associated 
> with the subnet.
> So, for such VM's, does one need to statically configure the IP 
> address with whatever Neutron has allocated from the pool?
> What exactly is the use case of the above?
> I do understand that for providing public network access to VM's, an 
> external network is generally created with enable-dhcp option set to 
> False. Is it only for this purpose?
> I was thinking of a case of external/provider DHCP servers from where 
> VM's can get their IP addresses and when one does not want to use L3 
> agent/DVR. In such cases, one may want to disable DHCP when creating 
> networks.  Isn't this a use-case?
> Appreciate any response or corrections with my above understanding.
> Thanks,
> Paddu
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John Kasperski

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