pandas dataframe comparing previous rows with conditions

Python and Pandas newbie here. I have the following dataframe and I would like to be able compare rows where the route and vals are the same in the previous row/rows and update the frm and to accordingly.

DF:
   route  frm    to  val
0      1    0   100    3
1      1  100   300    2
2      1  300   500    3
3      1  500  9999    3
4      2    0   100    3
5      2  100   300    3
6      2  300   500    3
7      2  500  9999    3

Desired Output:
   route  frm    to  val
0      1    0   100    3
1      1  100   300    2
3      1  300  9999    3
7      2  0    9999    3

I have tried the following using shift() which gets me some part of the way, but I am not sure the best way on how to achieve the desired output.

Any suggestions would be appreciated.

df['f'] = np.where((df.route.eq(df.route.shift())) & (df.val == df.val.shift()),df.frm.shift(),df.frm)
df['t'] = np.where((df.route.eq(df.route.shift())) & (df.val == df.val.shift()),df.to.shift(),df.to)

Output:

   route  frm    to  val    f    t
0      1    0   100    3    0  100
1      1  100   300    2  100  300
2      1  300   500    3  300  500
3      1  500  9999    3  300  500
4      2    0   100    3    0  100
5      2  100   300    3    0  100
6      2  300   500    3  100  300
7      2  500  9999    3  300  500

source https://stackoverflow.com/questions/74776651/pandas-dataframe-comparing-previous-rows-with-conditions

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