'Bring link(s) between two nodes up or down.
Usage: link node1 node2 [up/down]'

def do_link(self, line):

    args = line.split()
    if (len(args) != 3):
        error('invalid    number    of    args:    link    end1    end2    [up    down]\n')
    elif (args[2] not in ['up', 'down']):
        error('invalid    type:    link    end1    end2    [up    down]\n')
    else:
        self.mn.configLinkStatus(*args)
