** Also affects: neutron (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: neutron (Ubuntu Xenial)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1748894

Title:
  intermittent dhcp failures

Status in neutron:
  Incomplete
Status in neutron package in Ubuntu:
  New
Status in neutron source package in Xenial:
  New

Bug description:
  Hi,

  We have observed instances failing to get a DHCP reply, either when booting 
for the first time, or after a reboot.
  By tcpdumping the traffic on the tap, then qvb and qvo interfaces, we can see 
the DHCP request leaving, but it doesn't reach the neutron-gateway node where 
the dhcp agent for the network runs.

  If we remove and then re-add the network to the dhcp agent, it starts
  working fine again.

  I have dumped the output of `ovs-ofctl dump-flows br-tun` on the 
neutron-gateway side, before and after doing the remove/add.
  One difference I noted, was a rule appearing, with the mac of the VM which 
had the issue:
   cookie=0xb42e8d404b989e72, duration=13.344s, table=20, n_packets=0, 
n_bytes=0, hard_timeout=300, idle_age=13, 
priority=1,vlan_tci=0x0001/0x0fff,dl_dst=fa:16:3e:db:34:a3 
actions=load:0->NXM_OF_VLAN_TCI[],load:0x20->NXM_NX_TUN_ID[],output:5201

  However, this is a rule for packets from the dhcp agent to the VM, so
  its absence doesn't explain why DHCP requests don't arrive to
  destination.

  The cloud is running neutron 8.4.0 (Mitaka), on ubuntu (16.04)
  machines.

  Most of the time it works, but this problem comes back often enough
  that it impacts users.

  Thanks.

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