[ovs-dev] [PATCH] FAQ: Describe how to connect bridges with patch ports.

[Ben Pfaff]

I keep seeing this question.

Signed-off-by: Ben Pfaff <blp at nicira.com>
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 FAQ |   37 +++++++++++++++++++++++++++++++++++++
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diff --git a/FAQ b/FAQ
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--- a/FAQ
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@@ -325,6 +325,43 @@ A: No.  ERSPAN is an undocumented proprietary protocol.  As an
    alternative, Open vSwitch supports mirroring to a GRE tunnel (see
    above).
 
+Q: How do I connect two bridges?
+
+A: First, why do you want to do this?  Two connected bridges are not
+   much different from a single bridge, so you might as well just have
+   a single bridge with all your ports on it.
+
+   If you still want to connect two bridges, you can use a pair of
+   patch ports.  The follow example creates bridges br0 and br1, adds
+   eth0 and tap0 to br0, adds tap1 to br1, and then connects br0 and
+   br1 with a pair of patch ports.
+
+       ovs-vsctl add-br br0
+       ovs-vsctl add-port br0 eth0
+       ovs-vsctl add-port br0 tap0
+       ovs-vsctl add-br br1
+       ovs-vsctl add-port br1 tap1
+       ovs-vsctl \
+           -- add-port br0 patch0 \
+           -- set interface patch0 type=patch options:peer=patch1 \
+           -- add-port br1 patch1 \
+           -- set interface patch1 type=patch options:peer=patch0
+
+   Bridges connected with patch ports are much like a single bridge.
+   For instance, if the example above also added eth1 to br1, and both
+   eth0 and eth1 happened to be connected to the same next-hop switch,
+   then you could loop your network just as you would if you added
+   eth0 and eth1 to the same bridge (see the "Configuration Problems"
+   section below for more information).
+
+   If you are using Open vSwitch 1.9 or an earlier version, then you
+   need to be using the kernel module bundled with Open vSwitch rather
+   than the one that is integrated into Linux 3.3 and later, because
+   Open vSwitch 1.9 and earlier versions need kernel support for patch
+   ports.  This also means that in Open vSwitch 1.9 and earlier, patch
+   ports will not work with the userspace datapath, only with the
+   kernel module.
+
 Q: Why are there so many different ways to dump flows?
 
 A: Open vSwitch uses different kinds of flows for different purposes:
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