Added support for dual-homed hosts (on "classic" topo).

Change-Id: I47f4b3bf5756928452cbf99c4be2e3e1d6c8fa92
(cherry picked from commit 12c79edc912cce20596aa0cadc653ccd4d557cd8)
diff --git a/tools/test/topos/dual.py b/tools/test/topos/dual.py
new file mode 100644
index 0000000..f0d7524
--- /dev/null
+++ b/tools/test/topos/dual.py
@@ -0,0 +1,35 @@
+#!/usr/bin/env python
+
+"""
+"""
+from mininet.topo import Topo
+
+class DualTopo( Topo ):
+    """Switches and Dual-homed host"""
+
+    def __init__( self ):
+        """Create a topology."""
+
+        # Initialize Topology
+        Topo.__init__( self )
+
+        # add nodes, switches first...
+        LONDON = self.addSwitch( 's1' )
+        BRISTL = self.addSwitch( 's2' )
+
+        # ... and now hosts
+        LONDON_host = self.addHost( 'h1' )
+
+        # add edges between switch and corresponding host
+        self.addLink( LONDON, LONDON_host )
+        self.addLink( BRISTL, LONDON_host )
+
+        # add edges between switches
+        self.addLink( LONDON, BRISTL, bw=10, delay='1.0ms')
+
+
+topos = { 'dual': ( lambda: DualTopo() ) }
+
+if __name__ == '__main__':
+    from onosnet import run
+    run( DualTopo() )