I need to know if I must configure AARgroup in both DN's (remote and central site dn's) or if aar works if I configure aargroup in remote side only.
Yes, once you have more then 2 sites then you can still be fine with a single AAR group as long as the prefix between the sites is the same i.e. 91.
If you have 3 sites for example and between site A and B you dial 10 digits not 11, but between sites A and C and B and C you dial 11 digits then you have to create 2 AAR Groups:
prefix within group 9
prefix to AAR_11_Digits 91
prefix within group 91
prefix to AAR_10_Digits 91
the all DNs in site A and B will use AAR_10_Digits and DNs at site C will use group AAR_11_Digits
HTH, please rate all posts!