Hello Muhammad,

if I've understood correctly your scenario:

you have an HQ and some remote sites.

Inter-site connections are made using two MPLS Service Providers.

You have an MPLS L3 VPN and each of your router is a CE node with two eBGP sessions one to ISP1 and one to ISP2.

You would like to divide your traffic based on criteria beyond simple destination address.

For example you would like to send and receive Voip traffic via ISP1.

PBR is the right tool but your set action has to be

set ip next-hop ISP1-ipaddress

as-path prepending is effective only on route-maps applied to a neighbor under router bgp process.

if you did so you made one path more attractive for all possible destinations and only one provider is used to reach it.

If you have distinct ip subnets for Voip you can use prepending but it has to be selective.

! voip subnet

access-list 11 permit 10.10.100.0 0.0.0.255

route-map selective_prep_toISP2 permit 10

match ip address 11

set as-path-prepend yourASN yourASN

! empty final block to advertise all

! other ip subnets without prepending

route-map selective_prep_toISP2 permit 20

router bgp yourASN

neigh isp2.ipaddress route-map selective_prep_toISP2 out

a similar but mirrored ACL and route-map is needed for ISP1.

ISP2 receiving prepended AS path for net 10.10.100.0/24 shouldn't be used on the other site.

Edit:

depending on provider configuration it may be changing or removing your ASN using private-as-override or other command.

if so prepending may be not effective.

Hope to help

Giuseppe