Services and agents

A usual neutron setup consists of multiple services and agents running on one or multiple nodes (though some setups may not need any agents). Each of these services provide some of the networking or API services. Among those of special interest are:

The neutron-server that provides API endpoints and serves as a single point of access to the database. It usually runs on the controller nodes.
Layer2 agent that can utilize Open vSwitch, Linux Bridge or other vendor-specific technology to provide network segmentation and isolation for project networks. The L2 agent should run on every node where it is deemed responsible for wiring and securing virtual interfaces (usually both compute and network nodes).
Layer3 agent that runs on network node and provides east-west and north-south routing plus some advanced services such as FWaaS or VPNaaS.

Configuration options

The neutron configuration options are segregated between neutron-server and agents. Both services and agents may load the main neutron.conf since this file should contain the oslo.messaging configuration for internal neutron RPCs and may contain host specific configuration, such as file paths. The neutron.conf contains the database, keystone, nova credentials, and endpoints strictly for neutron-server to use.

In addition, neutron-server may load a plugin-specific configuration file, yet the agents should not. As the plugin configuration is primarily site wide options and the plugin provides the persistence layer for neutron, agents should be instructed to act upon these values through RPC.

Each individual agent may have its own configuration file. This file should be loaded after the main neutron.conf file, so the agent configuration takes precedence. The agent-specific configuration may contain configurations which vary between hosts in a neutron deployment such as the local_ip for an L2 agent. If any agent requires access to additional external services beyond the neutron RPC, those endpoints should be defined in the agent-specific configuration file (for example, nova metadata for metadata agent).

Agent's admin state specific config options

When creating a new agent the admin_state_up field will be set to the value of enable_new_agents config option, the default value of this config option is true:

[DEFAULT]
enable_new_agents = true

It is possible to set the admin_state_up value of an agent to False via the API, or CLI:

$ openstack network agent set agent-uuid --disable

The effect of this varies by agent type:

L2 agents

The admin_state_up field of the agent in the Neutron database is set to False, but the agent is still capable of binding ports. This is true for openvswitch-agent, linuxbridge-agent, and sriov-agent.

Note

In case of OVN based deployment Neutron doesn't keep track of OVN controllers in the agents db table, so setting the admin_state_up is not allowed as Neutron has no control over OVN entities. The possiblity to delete an OVN agent via Neutron REST API, is to clean up bad chassis information.

Metadata agent

Setting admin_state_up to False has no effect to the Metadata agent.

DHCP agent

DHCP agent scheduler will schedule networks to agents whose admin_state_up is True.

L3 agent

L3 scheduler will schedule routers to L3 agents whose admin_state_up field is True.

External processes run by agents

Some neutron agents, like DHCP, Metadata or L3, often run external processes to provide some of their functionalities. It may be keepalived, dnsmasq, haproxy or some other process. Neutron agents are responsible for spawning and killing such processes when necessary. By default, to kill such processes, agents use a simple kill command, but in some cases, like for example when those additional services are running inside containers, it may be not a good solution. To address this problem, operators should use the AGENT config group option kill_scripts_path to configure a path to where kill scripts for such processes live. By default, it is set to /etc/neutron/kill_scripts/. If option kill_scripts_path is changed in the config to the different location, exec_dirs in /etc/rootwrap.conf should be changed accordingly. If kill_scripts_path is set, every time neutron has to kill a process, for example dnsmasq, it will look in this directory for a file with the name <process_name>-kill. So for dnsmasq process it will look for a dnsmasq-kill script. If such a file exists there, it will be called instead of using the kill command.

Kill scripts are called with two parameters:

<process>-kill <sig> <pid>

where: <sig> is the signal, same as with the kill command, for example 9 or SIGKILL; and <pid> is pid of the process to kill.

This external script should then handle killing of the given process as neutron will not call the kill command for it anymore.

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