- <p>Open vSwitch permits a bridge to have any number of OpenFlow
- controllers. When multiple controllers are configured, Open vSwitch
- connects to all of them simultaneously. OpenFlow 1.0 does not specify
- how multiple controllers coordinate in interacting with a single switch,
- so more than one controller should be specified only if the controllers
- are themselves designed to coordinate with each other.</p>
+ <p>
+ Open vSwitch supports two kinds of OpenFlow controllers:
+ </p>
+
+ <dl>
+ <dt>Primary controllers</dt>
+ <dd>
+ <p>
+ This is the kind of controller envisioned by the OpenFlow 1.0
+ specification. Usually, a primary controller implements a network
+ policy by taking charge of the switch's flow table.
+ </p>
+
+ <p>
+ Open vSwitch initiates and maintains persistent connections to
+ primary controllers, retrying the connection each time it fails or
+ drops. The <ref table="Bridge" column="fail_mode"/> column in the
+ <ref table="Bridge"/> table applies to primary controllers.
+ </p>
+
+ <p>
+ Open vSwitch permits a bridge to have any number of primary
+ controllers. When multiple controllers are configured, Open
+ vSwitch connects to all of them simultaneously. Because
+ OpenFlow 1.0 does not specify how multiple controllers
+ coordinate in interacting with a single switch, more than
+ one primary controller should be specified only if the
+ controllers are themselves designed to coordinate with each
+ other. (The Nicira-defined <code>NXT_ROLE</code> OpenFlow
+ vendor extension may be useful for this.)
+ </p>
+ </dd>
+ <dt>Service controllers</dt>
+ <dd>
+ <p>
+ These kinds of OpenFlow controller connections are intended for
+ occasional support and maintenance use, e.g. with
+ <code>ovs-ofctl</code>. Usually a service controller connects only
+ briefly to inspect or modify some of a switch's state.
+ </p>
+
+ <p>
+ Open vSwitch listens for incoming connections from service
+ controllers. The service controllers initiate and, if necessary,
+ maintain the connections from their end. The <ref table="Bridge"
+ column="fail_mode"/> column in the <ref table="Bridge"/> table does
+ not apply to service controllers.
+ </p>
+
+ <p>
+ Open vSwitch supports configuring any number of service controllers.
+ </p>
+ </dd>
+ </dl>
+
+ <p>
+ The <ref column="target"/> determines the type of controller.
+ </p>