Add note about Linux Bridge MTU Bug w/ VXLAN tun

There's a horrible hard-to-diagnose bug when using vxlan tunelling.

This patch adds a note including the simple workaround.

Change-Id: I20a8ec46a91c7ffe1533338de5968d1b83badb6f
Closes-Bug: 1242534

doc/config-reference/networking/section_networking-plugins-ml2.xml

@@ -8,10 +8,41 @@
     xmlns:ns3="http://www.w3.org/1998/Math/MathML"
     xmlns:ns="http://docbook.org/ns/docbook" version="5.0">
     <title>Modular Layer 2 (ml2) configuration options</title>
-    <para>The Modular Layer 2 (ml2) plug-in has two components,
-        network types and mechanisms, that can be configured
-        separately. Such configuration options are described in the
-        subsections.</para>
+    <para>The Modular Layer 2 (ml2) plug-in has two components:
+        network types and mechanisms. You can configure these
+        components separately. This section describes these
+        configuration options.</para>
+    <note>
+        <title>MTU bug with VXLAN tunnelling</title>
+        <para>Due to a bug in Linux Bridge software maximum
+            transmission unit (MTU) handling, using VXLAN tunnels does
+            not work by default.</para>
+        <itemizedlist>
+            <listitem>
+                <para>A simple workaround is to increase the MTU value
+                    of the physical interface and physical switch
+                    fabric by at least 50 bytes. For example, increase
+                    the MTU value to 1550. This value enables an
+                    automatic 50-byte MTU difference between the
+                    physical interface (1500) and the VXLAN interface
+                    (automatically 1500-50 = 1450). An MTU value of
+                    1450 causes issues when virtual machine taps are
+                    configured at an MTU value of 1500.</para>
+            </listitem>
+            <listitem>
+                <para>Another workaround is to decrease the virtual
+                    ethernet devices' MTU. Set the
+                        <option>network_device_mtu</option> option to
+                    1450 in the <filename>neutron.conf</filename>
+                    file, and set all guest virtual machines' MTU to
+                    the same value by using a DHCP option. For
+                    information about how to use this option, see
+                        <link
+                        xlink:href="http://docs.openstack.org/admin-guide-cloud/content/ch_networking.html#openvswitch_plugin"
+                        >Configure OVS plug-in</link>.</para>
+            </listitem>
+        </itemizedlist>
+    </note>
     <xi:include href="../../common/tables/neutron-ml2.xml"/>
     <section xml:id="networking-plugin-ml2_flat">
         <title>Modular Layer 2 (ml2) Flat Type configuration