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I believe post
is addressing both questions:-

1.When the outside world wants to contact the VM’s floating IP, the FIP namespace will reply that is available via the fg’s device MAC address (An awful lie, but a useful one… Such is the life of a proxy). The traffic will be forwarded to the machine, in through a NIC connected to br-ex and in to the FIP’s namespace ‘fg’ device. The FIP namespace will use its route to and route it out its fpr veth device. The message will be received by the qrouter namespace: is configured on its rfp device, its iptables rules will replace the packet’s destination IP with the VM’s fixed IP of and off to the VM the message goes.

2. Legacy routers provide floating IPs connectivity by performing 1:1 NAT between the VM’s fixed IP and its floating IP inside the router namespace. Additionally, the L3 agent throws out a gratuitous ARP when it configures the floating IP on the router’s external device. This is done to advertise to the external network that the floating IP is reachable via the router’s external device’s MAC address. Floating IPs are configured as /32 prefixes on the router’s external device and so the router answers any ARP requests for these addresses. Legacy routers are of course scheduled only on a select subgroup of nodes known as network nodes