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[OpenStack Infra <1766380@xxxxxxxxxxxxxxxxxx>]

Reviewed:  https://review.openstack.org/573897
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=8361b8b5aebad4df3c1012952d9a87b936fef326
Submitter: Zuul
Branch:    master

commit 8361b8b5aebad4df3c1012952d9a87b936fef326
Author: Harald Jensås <hjensas@xxxxxxxxxx>
Date:   Sat Jun 9 02:46:56 2018 +0200

    Routed Networks - peer-subnet/segment host-routes (2/2)
    
    Ensure that host routes are maintained for each subnet within
    a network. Subnets associated with different segments on the
    same network get host_routes entries added/removed as subnets
    are created, deleted or updated.
    
    This change handle the host_routes for the peer subnets on the
    same network when a subnet is created or deleted.
    
    Also adds a shim api extension.
    
    APIImpact: Host routes are now calculated for routed networks.
    Closes-Bug: #1766380
    Change-Id: Iafbabe6352283e7f1a535a7b147bd81fb32f0ed1


** Changed in: neutron
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1766380

Title:
  [RFE] Create host-routes for routed networks (segments)

Status in neutron:
  Fix Released

Bug description:
  When using routed networks[1] on an instance connected to multiple
  networks the traffic from a segment_a to segment_b within a L3 network
  might be routed via a different network if the default router/gateway
  is not on the interface connecting to the routed network.

  It would be good to (at-least have an option to) automatically
  configure host_routes on the subnets in a routed L3 network. In such a
  way that traffic with a destination on a different segment within the
  same L3 network is routed via the instance interface connecting to the
  same L3 network.

  Example:
   instance_a:
     - port_a: some_net, segmentX, some_net_subnet  <-- default gateway
     - port_b: net1, segmentA, subnetA

   instance_b:
     - port_a: other_net, segmentY, other_net_subnet  <-- default gateway
     - port_b: net1, segmentB, subnetB

  Unless a host-route is in place, traffic from instance_a to instance_b
  will use some/other-net, not net1 which both is connected to.

  This RFE is to have the host_routes property on the subnets withing
  net1 populated, so that clients are aware of neighbour L3 networks.

  An example configuration:
  -------------------------

  First Create some_net and other_net:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  $ openstack network create \
    --provider-network-type flat \
    --provider-physical-network some_phys_net some_net
  $ openstack network create \
    --provider-network-type flat \
    --provider-physical-network other_phys_net other_net
  $ SOME_NET_SEGMENT_ID=$(openstack network segment list \
                            --network some_net -f value -c ID)
  $ OTHER_NET_SEGMENT_ID=$(openstack network segment list \
                             --network other_net -f value -c ID)
  $ openstack subnet create \
    --network-segment $SOME_NET_SEGMENT_ID \
    --network some_net \
    --ip-version 4 \
    --subnet-range 192.168.20.0/24 \
    --dhcp \
    --gateway 192.168.20.1 \
    some_net_subnet
  $ openstack subnet create \
    --network-segment $OTHER_NET_SEGMENT_ID \
    --network other_net \
    --ip-version 4 \
    --subnet-range 192.168.30.0/24 \
    --dhcp \
    --gateway 192.168.30.1 \
    other_net_subnet

  Create net1, with two segment (segmentA and segmentB):
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  $ openstack network create \
    --provider-network-type flat \
    --provider-physical-network net1_segmentA \
    net1
  $ NET1_SEGMENTA_ID=$(openstack network segment list \
                         --network net1 -f value -c ID)
  $ openstack network segment set --name net1_segmentA $NET1_SEGMENTA_ID
  $ openstack network segment create \
    --network-type flat \
    --physical-network net1_segmentB \
    --network net1 \
    net1_segmentB

  Create two subnets associated with different segments on net1:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  $ openstack subnet create \
    --network-segment net1_segmentA \
    --network net1 \
    --ip-version 4 \
    --subnet-range 192.168.100.0/25 \
    --dhcp \
    subnetA
  $ openstack subnet create \
    --network-segment net1_segmentB \
    --network net1 \
    --ip-version 4 \
    --subnet-range 192.168.100.128/25 \
    --dhcp \
    subnetB

  <pseudo-command> $ openstack server create \
                       --network some_network \
                       --netowork net1 \
                       instance_a

  <pseudo-command> $ openstack server create \
                       --network other_network \
                       --netowork net1 \
                       instance_b

  NOTE: Assume instance_a is scheduled on a compute with bridge mapping
  to physical network net1_segmentA, and instance_b is scheduled to a
  compute with bridge mapping to physical network net1_segmentB.

  What we end up with is traffic within net1 (the routed network) is
  routed via the default gateway on some_net/other_net.

  The cloud user most likely does not know that net1 is a routed
  network, and expect traffic within net1 _not to be_ routed via some
  /other-net.

  To solve this problem neutron should do the equivalent of:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  $ openstack subnet set \
    --host-route destination=192.168.100.128/25,gateway=192.168.100.1 \
    subnetA
  $ openstack subnet set \
    --host-route destination=192.168.100.0/25,gateway=192.168.100.129 \
    subnetB

  ^^ Would make DHCP agents advertise the routes within net1 to clients,
  making traffic destined for other segments on net1 take a path via a
  route within net1 (within the routed network).

  I believe it would make sense to automate this, so that when
  additional subnets on additional segments are added the new
  destination is appended to the host routes.

  [1] https://docs.openstack.org/neutron/latest/admin/config-routed-
  networks.html

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