[Bug 1887148] Re: Network loop between physical networks with DVR

[Edward Hope-Morley]

All SRU verification completed and performed in
https://bugs.launchpad.net/neutron/+bug/1869808 so please refer to that
LP for the results.

** Tags removed: verification-needed verification-needed-bionic verification-queens-needed verification-rocky-needed
** Tags added: verification-done verification-done-bionic verification-queens-done verification-rocky-done

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https://bugs.launchpad.net/bugs/1887148

Title:
  Network loop between physical networks with DVR

Status in Ubuntu Cloud Archive:
  Invalid
Status in Ubuntu Cloud Archive queens series:
  Fix Committed
Status in Ubuntu Cloud Archive rocky series:
  Fix Committed
Status in neutron:
  Fix Released
Status in neutron package in Ubuntu:
  Fix Released
Status in neutron source package in Bionic:
  Fix Committed

Bug description:
  (For SRU template, please see bug 1869808, as the SRU info there
  applies to this bug also)

  
  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 at q-svc
  systemctl stop devstack at q-agt

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

  # start agent
  systemctl start devstack at 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 one 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

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