Minimizing Network Connectivity Downtime on Host Reboot

When a host with a DVR router reboots, there is a temporary loss of network connectivity from the host to the provider network or the external network. The loss of network connectivity occurring due to host reboot or downtime can be minimized by manually moving the DVR router to another host.

In order to manually move or reschedule DVR routers from the current host to the new host, you must reassign the SNAT agent from the current host to the new host.

Follow the steps given below to manually reassign the SNAT agent from one host to another. Repeat the process for each router associated with the host that is rebooting.

  1. Get a list of all hosts running Neutron L3 agents to find another SNAT-capable L3 agent.
    openstack network agent list
  2. Look for an agent with the agent_mode setting as dvr_snat.
    Note: If this agent is set only for dvr, it is not  SNAT-capable. This is configured when you set up the host.
    Confirm the agent_mode setting with the following command.
    openstack network agent show <l3_agent_uuid>​
  3. Identify the L3 agent that currently provides centralized SNAT for the given router. You can get a list of routers with the openstack router list command. You can then identify the routers associated with the host that is rebooting.
    openstack router list
    openstack port list --device-owner network:router_centralized_snat --router <router_uuid>
    openstack port show <port_uuid> --column binding_host_id --format value
  4. Unassign the SNAT agent from the host.
    neutron l3-agent-router-remove <l3_agent_uuid> <router_uuid>​ 

  5. Assign the SNAT agent to one of the SNAT capable L3-agents found in step 2.
    neutron l3-agent-router-add <new_l3_agent_uuid> <router_uuid>​​

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