def METHOD_NAME(tgen):
    "Build function"
    # Create 3 routers
    for routern in range(1, 4):
        tgen.add_router("r{}".format(routern))
    switch = tgen.add_switch("s1")
    switch.add_link(tgen.gears["r1"])
    switch.add_link(tgen.gears["r2"])
    switch = tgen.add_switch("s2")
    switch.add_link(tgen.gears["r2"])
    switch.add_link(tgen.gears["r3"])
    # Create stub networks for multicast traffic.
    tgen.add_host("h1", "192.168.10.2/24", "via 192.168.10.1")
    switch = tgen.add_switch("s3")
    switch.add_link(tgen.gears["r1"])
    switch.add_link(tgen.gears["h1"])
    tgen.add_host("h2", "192.168.30.2/24", "via 192.168.30.1")
    switch = tgen.add_switch("s4")
    switch.add_link(tgen.gears["r3"])
    switch.add_link(tgen.gears["h2"])