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======================================
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Live migrate Neutron routers (OVS/ML2)
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======================================
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Introduction
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------------
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Neutron routers are hosted by ``L3 Agents`` running on `neutron-gateway`_
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units. When the unit(s) need to be brought down for maintenance, routers need
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to be migrated to another unit that will host them during the downtime.
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.. note::
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This tutorial deals with migration of a router strictly in the OVS/ML2
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environment, deployments using OVN have different topology in which routers
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are not hosted by individual ``L3 Agents``.
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.. warning::
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Migration of a router will cause brief disruption of a traffic between
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networks connected to the router.
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Example topology
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----------------
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To demonstrate steps needed for the migration, we will use openstack deployment
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with three `neutron-gateway`_ units:
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.. code-block:: console
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Unit Workload Agent Machine Public address Ports Message
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neutron-gateway/0* active idle 0 10.5.1.149 Unit is ready
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ntp/0* active idle 10.5.1.149 123/udp chrony: Ready
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neutron-gateway/1 active idle 13 10.5.2.17 Unit is ready
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ntp/4 active idle 10.5.2.17 123/udp chrony: Ready
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neutron-gateway/2 active idle 14 10.5.0.159 Unit is ready
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ntp/5 active idle 10.5.0.159 123/udp chrony: Ready
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Machine State DNS Inst id Series AZ Message
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0 started 10.5.1.149 1bf707d8-3e27-473f-b597-5cfc43c50b85 bionic nova ACTIVE
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13 started 10.5.2.17 960d3868-a5a3-443c-99a8-acbcec7a4272 bionic nova ACTIVE
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14 started 10.5.0.159 c8c75144-7a5b-49c0-bfb5-4910c6501f21 bionic nova ACTIVE
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Each of these units hosts single ``L3 Agent``:
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.. code-block:: console
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openstack network agent list --agent-type l3
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| ID | Agent Type | Host | Availability Zone | Alive | State | Binary |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| 6226d159-ed57-4dfb-a311-ae36d2b4462b | L3 agent | juju-56ea63-os-0 | nova | :-) | UP | neutron-l3-agent |
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| 6ae7ed32-7aa4-460c-8eb7-d13f4e651fcc | L3 agent | juju-56ea63-os-13 | nova | :-) | UP | neutron-l3-agent |
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| b066c34d-aa91-46fe-9b20-f19f4a297932 | L3 agent | juju-56ea63-os-14 | nova | :-) | UP | neutron-l3-agent |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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And we will be bringing down unit ``neutron-gateway/2`` for maintenance.
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Procedure
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---------
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Find out which routers run on our targeted unit
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Referencing our example topology above, we are about to bring down unit
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``neutron-gateway/2``. We need to find out which routers are hosted on it.
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We can see that this unit is running on machine ``14``, listing all the
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L3 Agents we get:
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.. code-block:: console
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openstack network agent list --agent-type l3
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| ID | Agent Type | Host | Availability Zone | Alive | State | Binary |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| 6226d159-ed57-4dfb-a311-ae36d2b4462b | L3 agent | juju-56ea63-os-0 | nova | :-) | UP | neutron-l3-agent |
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| 6ae7ed32-7aa4-460c-8eb7-d13f4e651fcc | L3 agent | juju-56ea63-os-13 | nova | :-) | UP | neutron-l3-agent |
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| b066c34d-aa91-46fe-9b20-f19f4a297932 | L3 agent | juju-56ea63-os-14 | nova | :-) | UP | neutron-l3-agent |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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Machine ``14`` corresponds to Host ``juju-56ea63-os-14``, so we need to find
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out which routers are hosted on the ``L3 agent`` with ID
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``b066c34d-aa91-46fe-9b20-f19f4a297932``.
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.. code-block:: console
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openstack router list --agent b066c34d-aa91-46fe-9b20-f19f4a297932
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+--------------------------------------+-----------------+--------+-------+-------------+-------+----------------------------------+
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| ID | Name | Status | State | Distributed | HA | Project |
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+--------------------------------------+-----------------+--------+-------+-------------+-------+----------------------------------+
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| 35dcce1b-f69f-4af3-b46f-54249812ec9f | provider-router | ACTIVE | UP | False | False | b800e60c5e3841fbbb8b1dbc02ce13e3 |
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+--------------------------------------+-----------------+--------+-------+-------------+-------+----------------------------------+
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This means that we need to move router with name ``provider-router`` to a
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different agent.
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Move router to a different agent
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This is the part that will cause brief disruption of a traffic as routers can
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not be moved seamlessly. Moving consists of manually removing router from one
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agent and adding it to another. First we double-check that the router is hosted
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on the agent that is about to go down:
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.. code-block:: console
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openstack network agent list --router provider-router
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| ID | Agent Type | Host | Availability Zone | Alive | State | Binary |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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| b066c34d-aa91-46fe-9b20-f19f4a297932 | L3 agent | juju-56ea63-os-14 | nova | :-) | UP | neutron-l3-agent |
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+--------------------------------------+------------+-------------------+-------------------+-------+-------+------------------+
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Now we remove the router from this agent and assign it to an agent running on
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the unit ``neutron-gateway/0`` with agent ID
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``6226d159-ed57-4dfb-a311-ae36d2b4462b``. Note that this will increase the load
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on the ``neutron-gateway/0`` unit. Make sure that the machine hosting this unit
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has enough resources to accommodate the additional router.
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.. code-block:: console
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openstack network agent remove router b066c34d-aa91-46fe-9b20-f19f4a297932 provider-router --l3
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openstack network agent add router 6226d159-ed57-4dfb-a311-ae36d2b4462b provider-router --l3
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Result of this move can be checked by:
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.. code-block:: console
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openstack network agent list --router provider-router
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+--------------------------------------+------------+------------------+-------------------+-------+-------+------------------+
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| ID | Agent Type | Host | Availability Zone | Alive | State | Binary |
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+--------------------------------------+------------+------------------+-------------------+-------+-------+------------------+
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| 6226d159-ed57-4dfb-a311-ae36d2b4462b | L3 agent | juju-56ea63-os-0 | nova | :-) | UP | neutron-l3-agent |
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+--------------------------------------+------------+------------------+-------------------+-------+-------+------------------+
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.. LINKS
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.. _neutron-gateway : https://charmhub.io/neutron-gateway
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Martin Kalcok