[CORD-2940] Docs update

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+# Fabric service
+
+The Fabric service is responsible to configure the fabric switches in ONOS.
+
+At this point we assume you followed the
+[Fabric seutp](../prereqs/fabric-setup.md) guide and your physical infrastructure
+is configured and ready to go.
+
+## How to configure fabric switches
+
+Here is an example of how to configure a `Switch` a `Port` and an `Interface`
+on your fabric.
+
+```yaml
+tosca_definitions_version: tosca_simple_yaml_1_0
+imports:
+  - custom_types/switch.yaml
+  - custom_types/switchport.yaml
+  - custom_types/portinterface.yaml
+description: Configures fabric switches and related ports
+topology_template:
+  node_templates:
+    switch#leaf1:
+      type: tosca.nodes.Switch
+      properties:
+        driver: ofdpa3 # The driver used by the SDN controller
+        ipv4Loopback: 192.168.0.201 # Fabric loopback interface
+        ipv4NodeSid: 17 # The MPLS label used by the switch [17 - 1048576]
+        isEdgeRouter: True # Whether is a leaf (True) or a spine (False)
+        name: leaf1
+        ofId: of:0000000000000001 # The unique OpenFlow ID of the fabric switch
+        routerMac: 00:00:02:01:06:01 # MAC address of the fabric switch used for all interfaces
+
+    # Setup a fabric switch port
+    port#port1:
+      type: tosca.nodes.SwitchPort
+      properties:
+        portId: 1 # The unique port OpenFlow port ID
+      requirements:
+        - switch:
+            node: switch#leaf1
+            relationship: tosca.relationships.BelongsToOne
+
+    # Setup a fabric switch port interface
+    interface#interface1:
+      type: tosca.nodes.PortInterface
+      properties:
+        ips: 192.168.0.254/24 # The interface IP address (xxx.yyy.www.zzz/nm)
+        name: port1
+      requirements:
+        - port:
+            node: port#port1
+            relationship: tosca.relationships.BelongsToOne
+```