[ovn][l3-ha] BFD monitoring not working for network down

[Dongcan Ye]

OVN version: 21.12.1
OVS version: 2.15.6
OpenStack version: Yoga

In my test environment, two nodes as HV Chassis and enabled these chassis to host gateways.
1. Add a network and subnet.
2. Add a router, then attach subnet to router.
3. Set router's external gateway.
4. Binding a floatingip to an instance.

Then if we use 'ip link set dev *** down' to make the physical tunnel interface down. The floatingip can't access.

# ovn-nbctl list Logical_Router_Port lrp-95c640e8-7249-41ee-a407-98a2bc8741ca
_uuid : 2b3f9e79-33ee-45aa-9a23-ad9e9e460455
enabled : []
external_ids : {"neutron:network_name"=neutron-d1d2dcab-e701-4cc2-8c64-c7d81d5f33e5, "neutron:revision_number"="47", "neutron:router_name"="801a8e21-e368-4b9f-8119-830d7f743322", "neutron:subnet_ids"="a3bbe66c-f963-4337-ae8e-bb5fd3e7bde9"}
gateway_chassis : [11f482cd-18fc-42d4-a3a6-e036ae428371, dc861c0a-e8d6-4b98-ae7f-de0e418f3163]
ha_chassis_group : []
ipv6_prefix : []
ipv6_ra_configs : {}
mac : "fa:16:3e:38:5b:50"
name : lrp-95c640e8-7249-41ee-a407-98a2bc8741ca
networks : ["192.168.135.248/20"]
options : {}
peer : []

We can't see the gateway_chassis priority changes.

# ovn-nbctl list gateway_chassis
_uuid : 11f482cd-18fc-42d4-a3a6-e036ae428371
chassis_name : "d2692315-6389-4e95-b211-a3dd54ec4582"
external_ids : {}
name : lrp-95c640e8-7249-41ee-a407-98a2bc8741ca_d2692315-6389-4e95-b211-a3dd54ec4582
options : {}
priority : 2

_uuid : dc861c0a-e8d6-4b98-ae7f-de0e418f3163
chassis_name : "00b6dd97-6360-4ae5-b1f4-a68cf2e8c70c"
external_ids : {}
name : lrp-95c640e8-7249-41ee-a407-98a2bc8741ca_00b6dd97-6360-4ae5-b1f4-a68cf2e8c70c
options : {}
priority : 1

After read the HA guide in OVN[1], it said we can set cpath_down config.
But there is no effective:
# ovn-nbctl --wait=hv set NB_Global . options:"bfd-cpath-down"=true
# ovs-vsctl set interface ovn-d26923-0 bfd:cpath_down=true

In guide[2], we test three situations, it works for us:
(1) The gateway chassis is shutdown.
(2) The ovs-vswitchd is stopped.
(3) The ovn-controller is stopped.

[1] https://docs.ovn.org/en/latest/topics/high-availability.html
[2] https://docs.openstack.org/neutron/latest/admin/ovn/routing.html