<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def setup_link(self, interface, cidr):
 """Setup a link. ip addr add dev interface ip link set dev interface up """

        # clear old ipaddr in interface
        cmd = ['ip', 'addr', 'flush', 'dev', interface]
        agent_utils.execute(cmd, root=True)
        ip = IPNetwork(cidr)
        cmd = ['ip', 'addr', 'add', cidr, 'broadcast',
               str(ip.broadcast), 'dev', interface]
        stdcode, stdout = agent_utils.execute(cmd, root=True)
        if stdcode == 0:
            cmd = ['ip', 'link', 'set', 'dev', interface, 'up']
            stdcode, stdout = agent_utils.execute(cmd, root=True)
            if stdcode == 0:
                return agent_utils.make_response(code=stdcode)
        # execute failed.
        message = stdout.pop(0)
        return agent_utils.make_response(code=stdcode, message=message)