Install guide: talk about routing mode

The networking section is updated to talk about both routing and
bridging.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

doc/install.rst

@@ -326,17 +326,36 @@ Configuring the network
 
 **Mandatory** on all nodes.
 
-Ganeti relies on Xen running in "bridge mode", which means the
-instances network interfaces will be attached to a software bridge
-running in dom0. Xen by default creates such a bridge at startup, but
-your distribution might have a different way to do things.
+You can run Ganeti either in "bridge mode" or in "routed mode". In bridge
+mode, the default, the instances network interfaces will be attached to a
+software bridge running in dom0. Xen by default creates such a bridge at
+startup, but your distribution might have a different way to do things, and
+you'll definitely need to manually set it up under KVM.
 
 Beware that the default name Ganeti uses is ``xen-br0`` (which was
 used in Xen 2.0) while Xen 3.0 uses ``xenbr0`` by default. The default
 bridge your Ganeti cluster will use for new instances can be specified
 at cluster initialization time.
 
-.. admonition:: Debian
+If you want to run in "routing mode" you need to specify that at cluster init
+time (using the --nicparam option), and then no bridge will be needed. In
+this mode instance traffic will be routed by dom0, instead of bridged.
+
+In order to use "routing mode" under Xen, you'll need to change the relevant
+parameters in the Xen config file. Under KVM instead, no config change is
+necessary, but you still need to set up your network interfaces correctly.
+
+By default, under KVM, the "link" parameter you specify per-nic will
+represent, if non-empty, a different routing table name or number to use for
+your instances. This allows insulation between different instance groups,
+and different routing policies between node traffic and instance traffic.
+
+You will need to configure your routing table basic routes and rules outside
+of ganeti. The vif scripts will only add /32 routes to your instances,
+through their interface, in the table you specified (under KVM, and in the
+main table under Xen).
+
+.. admonition:: Bridging under Debian
 
    The recommended way to configure the Xen bridge is to edit your
    ``/etc/network/interfaces`` file and substitute your normal
@@ -493,6 +512,9 @@ that the hostname used for this must resolve to an IP address reserved
 **exclusively** for this purpose, and cannot be the name of the first
 (master) node.
 
+If you want to use a bridge which is not ``xen-br0``, or no bridge at all, use
+the --nicparams
+
 If the bridge name you are using is not ``xen-br0``, use the *-b
 <BRIDGENAME>* option to specify the bridge name. In this case, you
 should also use the *--master-netdev <BRIDGENAME>* option with the