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+.. raw:: html
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+   <br/>
+   <br/>
+
+.. _platform_routing:
+
+Defining a Routing
+##################
+
+Networking zones (:ref:`pf_tag_zone`) are an advanced concept used to factorize the description
+to reduce the size of your platform on disk and in memory. Then, when
+a host wants to communicate with another host belonging to the same
+zone, it is the zone's duty to find the list of links that are
+involved in the communication. In the above examples, since we use
+``routing="Full"``, all routes must be explicitly given using the
+:ref:`pf_tag_route` and :ref:`pf_tag_link_ctn` tags (this :ref:`routing
+model <pf_rm>` is both simple and inefficient :) It is OK to not
+specify each and every route between hosts, as long as you do not try
+to start a communication on any of the missing routes during your
+simulation.
+
+Any zone may contain sub-zones, allowing for a hierarchical
+decomposition of the platform. Routing can be made more efficient (as the
+inter-zone routing gets factored with :ref:`pf_tag_zoneroute`), and
+allows you to have more than one routing model in your platform. For
+example, you can have a coordinate-based routing for the WAN parts
+of your platforms, a full routing within each datacenter, and a highly
+optimized routing within each cluster of the datacenter.  In this
+case, determining the route between two given hosts gets :ref:`routing_basics`
+"somewhat more complex" but SimGrid still computes
+these routes for you in a time- and space-efficient manner.
+Here is an illustration of these concepts:
+
+.. image:: img/zone_hierarchy.png
+
+Circles represent processing units and squares represent network
+routers. Bold lines represent communication links. The zone "AS2" models the core of a national network interconnecting a
+small flat cluster (AS4) and a larger hierarchical cluster (AS5), a
+subset of a LAN (AS6), and a set of peers scattered around the world
+(AS7).
+
+.. todo:: Add more examples, such as the cloud example described in
+          previous paragraph
+