DHCP host_routes should be Neutron Segment aware

[Harald Jensås]

The following code in neutron/agent/linux/dhcp.py will create on-link/link-local routes to all subnets in a network.

 host_routes.extend(["%s,0.0.0.0" % (s.cidr) for s in
                                    self.network.subnets
                                    if (s.ip_version == 4 and
                                        s.cidr != subnet.cidr)])

White the introduction of Neutron segments, this needs to be update so that host_routes for on-link destinations are only created if l2-adjacency is True.

We should also create host_routes for all subnets whithin the same segment, as I understand it l2-adjacency is guaranteed within each segment. And Segment can have multiple Subnets.

ref: http://specs.openstack.org/openstack/neutron-specs/specs/newton/routed-networks.html

[Harald Jensås (harald-jensas)]

I wonder if the DHCP agent is the correct place to implement them. Hidden from the end user. I don't like it.

Would it not be better to add them as "host-routes" on the subnet in neutron?

[Kevin Benton (kevinbenton)]

@Harald, possibly, but we already have this behavior that we should fix before we figure out if it should be represented in the API.

[Harald Jensås (harald-jensas)]

This change closes this issue.

https://review.openstack.org/#/c/438171/