[OVN] Scheduling of HA Chassis Group for external port does not work when no chassis has 'enable-chassis-as-gw' option set

[Frode Nordahl]

The 'enable-chassis-as-gw' CMS option allows the operator to influence where L3 routers and HA Chassis Groups for external ports are scheduled.

When no chassis has the option set all chassis will be eligible for scheduling of L3 gateways [0].

This is a powerful concept, and it gives operators fine grained control when needed, and lets the system decide when the fine grained control is not needed.

However, this logic is not applied for HA Chassis Groups for use with the external port for servicing of SR-IOV instances. If no chassis has the 'enable-chassis-as-gw' option set, no HA Chassis Group for use with external ports are created, and subsequently SR-IOV instances will not be serviced for DHCP or Metadata.

While I understand the desire to require the use of the 'enable-chassis-as-gw' option and the simplification of the code that entails, it does introduce a unexpected difference in behavior from the scheduling of L3 gateways as documented in [0]. It also removes some of the power of the option.

For example, when you have a deployment in a L3-only DC fabric and do not use features such as EVPN, you most likely want to schedule gateways only to chassis in physical vicinity of your border gateways. If you have a deployment in a L3-only DC fabric with EVPN, you would most likely not care where the gateways are scheduled and would like the system to schedule the gateways for you. Other deployment scenarios are where you have a set of hypervisors you for some reason don't want any centralized services scheduled to them.

The current behavior of the HA Chassis Group scheduling only caters for the latter of the above scenarios, and makes it more cumbersome for the user to support the two former.

Would it be more consistent if Neutron chose among all chassis when no chassis has the 'enable-chassis-as-gw' option set also for the HA Chassis Group for external port feature?

0: https://docs.openstack.org/neutron/latest/admin/ovn/routing.html#l3ha-support