Comment 0 for bug 1887148

Our CI experienced a network loop due to https://review.opendev.org/#/c/733568/ . DVR is enabled and there is more than one physical bridge mapping, and the neutron server was not available when the ovs agents were started.

Steps
=====
# add more physical bridges
ovs-vsctl add-br br-physnet1
ip link set dev br-physnet1 up

ovs-vsctl add-br br-physnet2
ip link set dev br-physnet2 up

# set a broadcast going from one bridge
ip address add 1.1.1.1/31 dev br-physnet1
arping -b -I br-physnet1 1.1.1.1

# listen on the other
tcpdump -eni br-physnet2

# Update /etc/neutron/plugins/ml2/ml2_conf.ini
[ml2_type_vlan]
network_vlan_ranges = public,physnet1,physnet2

[ovs]
datapath_type = system
bridge_mappings = public:br-ex,physnet1:br-physnet1,physnet2:br-physnet2
tunnel_bridge = br-tun
local_ip = 127.0.0.1

[agent]
tunnel_types = vxlan
root_helper_daemon = sudo /usr/local/bin/neutron-rootwrap-daemon /etc/neutron/rootwrap.conf
root_helper = sudo /usr/local/bin/neutron-rootwrap /etc/neutron/rootwrap.conf
enable_distributed_routing = True
l2_population = True

# stop server and agent
systemctl stop devstack@q-svc
systemctl stop devstack@q-agt

# clear all flows
for BR in $(sudo ovs-vsctl list-br); do echo $BR; sudo ovs-ofctl del-flows $BR; done

systemctl start devstack@q-agt

$ sudo tcpdump -eni br-physnet2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br-physnet2, link-type EN10MB (Ethernet), capture size 262144 bytes
09:46:56.577183 e2:ab:d4:16:46:4d > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42: Request who-has 1.1.1.1 (ff:ff:ff:ff:ff:ff) tell 1.1.1.1, length 28
09:46:57.577568 e2:ab:d4:16:46:4d > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42: Request who-has 1.1.1.1 (ff:ff:ff:ff:ff:ff) tell 1.1.1.1, length 28
...

If there is more than node running the ovs agent in this state, then there will be a network loop and packets can multiple quickly and overwhelm the network. We saw ~1 million packets/sec.

I think because the neutron server is not available, the get_dvr_mac_address rpc is blocked and the required drops are not installed:
https://github.com/openstack/neutron/blob/master/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_dvr_neutron_agent.py#L138
https://github.com/openstack/neutron/blob/5999716cfc4a00ac426e016eabbb51247ba0b190/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_dvr_neutron_agent.py#L230-L234