def METHOD_NAME(tgen):
    "Build function"
    # This function's only purpose is to define allocation and relationship
    # between routers, switches and hosts.
    #
    #
    # Create routers
    tgen.add_router("R1")
    tgen.add_router("R2")
    tgen.add_router("R3")
    tgen.add_router("R4")
    # R1-R2
    switch = tgen.add_switch("s1")
    switch.add_link(tgen.gears["R1"])
    switch.add_link(tgen.gears["R2"])
    # R2-R3
    switch = tgen.add_switch("s2")
    switch.add_link(tgen.gears["R2"])
    switch.add_link(tgen.gears["R3"])
    # R1-R4
    switch = tgen.add_switch("s3")
    switch.add_link(tgen.gears["R1"])
    switch.add_link(tgen.gears["R4"])