.. _config-services-agent:

===================
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
``external_network_bridge`` for an L3 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).