How to add one data frame below another data frame without any identifiers

Question

I have to dfs that have the same column names in the exact same order like this

df1 <-

a  b  c  d
1  2  3  4
1  2  3  4
1  2  3  4
1  2  3  4


df2 <-
a  b  c  d
4  3  2  1
4  3  2  1
4  3  2  1
4  3  2  1

I would like to simply merge these dfs by adding df2 below df1 to get df3

df3 <- 

    a  b  c  d
    1  2  3  4
    1  2  3  4
    1  2  3  4
    1  2  3  4
    4  3  2  1
    4  3  2  1
    4  3  2  1
    4  3  2  1

I tried merge(df1,df2) but the result is an empty df.