Add documentation for Linux Bridge (previously missing)

Change-Id: I092b609f43b37ed85d08bc80d1d048b945abe222
Closes-Bug: #1455979

doc/source/devref/linuxbridge_agent.rst

@@ -1,2 +1,27 @@
+===============================
 L2 Networking with Linux Bridge
--------------------------------
+===============================
+
+This Agent uses the `Linux Bridge
+<http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge>`_ to
+provide L2 connectivity for VM instances running on the compute node to the
+public network.  A graphical illustration of the deployment can be found in
+`OpenStack Admin Guide Linux Bridge
+<http://docs.openstack.org/admin-guide-cloud/content/under_the_hood_linuxbridge.html>`_
+
+In most common deployments, there is a compute and a network node. On both the
+compute and the network node, the Linux Bridge Agent will manage virtual
+switches, connectivity among them, and interaction via virtual ports with other
+network components such as namespaces and underlying interfaces. Additionally,
+on the compute node, the Linux Bridge Agent will manage security groups.
+
+Three use cases and their packet flow are documented as follows:
+
+1. `Legacy implementation with Linux Bridge
+   <http://docs.openstack.org/networking-guide/deploy_scenario1b.html>`_
+
+2. `High Availability using L3HA with Linux Bridge
+   <http://docs.openstack.org/networking-guide/deploy_scenario3b.html>`_
+
+3. `Provider networks with Linux Bridge
+   <http://docs.openstack.org/networking-guide/deploy_scenario4b.html>`_