def METHOD_NAME(tgen):
    "Build function"
    for routern in range(1, 4):
        tgen.add_router("r{}".format(routern))
    tgen.add_router("rp")
    #   rp ------ r1 -------- r2
    #              \
    #               --------- r3
    # r1 -> .1
    # r2 -> .2
    # rp -> .3
    # r3 -> .4
    # loopback network is 10.254.0.X/32
    #
    # r1 <- sw1 -> r2
    # r1-eth0 <-> r2-eth0
    # 10.0.20.0/24
    sw = tgen.add_switch("sw1")
    sw.add_link(tgen.gears["r1"])
    sw.add_link(tgen.gears["r2"])
    # r1 <- sw2 -> rp
    # r1-eth1 <-> rp-eth0
    # 10.0.30.0/24
    sw = tgen.add_switch("sw2")
    sw.add_link(tgen.gears["r1"])
    sw.add_link(tgen.gears["rp"])
    # 10.0.40.0/24
    sw = tgen.add_switch("sw3")
    sw.add_link(tgen.gears["r1"])
    sw.add_link(tgen.gears["r3"])