Comment 0 for bug 1471637

Should support a configurable mode for VN. The 3 modes to be supported
are:

1. L2+ L3 mode: This is currently supported in 2.20 and newer releases and
will be the default mode for a VN. Intra subnet traffic is bridged and inter-
subnet traffic is routed.

2. L2 only mode: This is required for certain applications that need pure L2
forwarding capabilities e.g. Junosphere. Even though there may be a subnet
configured for the VN, the ports/VMIs do not use addresses from the subnet.
Proxy ARP is disabled and all ARP packets are flooded - the endpoints are
responsible for responding to ARP requests. Similarly, DHCP and DNS are
also disabled. The vRouter should not advertise any IP addresses in MAC
routes.

3. L3 only mode: This is the classic mode that was supported in 1.x releases
before BMS and and IRB support was implemented. The vRouter responds
to all ARP requests with the VRRP MAC address. DNS and DHCP are also
supported. The vRouter should not advertise any EVPN routes for MACs.

In addition to supporting a mode per VN, it might also be worth making it
configurable in the global system config and allowing the user to override
it on a per VN basis.