Comment 1 for bug 1046121

In a discussion over the maling list (or a review, can't remember) we made the point that an external network can be used, in some circustances, directly by instances. For instance an user might want to deploy its own router, or a load balancer, or any other appliance requiring a public and a private interface.

Keeping that in mind, we might want to still use dhcp on external networks. Even if they are directly connect on a segment outside of quantum scope, the dhcp server will only respond to requests coming from quantum vms - so we won't risk it distributing quantum addresses to the rest of the world.

The current process for associating a floating IP also ensures it is removed from the allocation pool, so VMs with VIF on the external network won't get it.

We can change the default behaviour for dhcp_enable. On way of doing this would be setting the default to ATTR_NOT_SPECIFIED, and then setting it in the plugin.

The db base plugin would set it to True if it is not specified, whereas plugins deriving from it and the l3 mixin should instead look at the value of router:external before making a decision.