Which of the following argument is used to ignore the index while concatenating two data frames? code example

Example 1: pd merge on multiple columns

new_df = pd.merge(A_df, B_df,  how='left', left_on=['A_c1','c2'], right_on = ['B_c1','c2'])

Example 2: merge on row number python

pd.concat([T1,T2],axis=1)