how to configure BGP routers to send traffic out to the peer from whom it received traffic originall

Topology

R1  - Has two Subnets (192.168.10.x) and (192.168.20.x) and Shares MPLS cloud provide by ISP.

R2 & R3 also shares the same MPLS cloud

MPLS cloud is running BGP with all three routers.

R2 and R3 has one serial interface connecting it back to MPLS cloud and another serial connection connecting back to R4.

R4 LAN segment (192.168.30.x), R4 has PBR on LAN interface that traffic destined to 192.168.10.x should route down to R2 and traffic destined to 192.168.20.x should route down to R3. Any client on that LAN can access any of the remote subnets.

Both R2 and R3 are advertising R4's LAN interface through BGP.

Due to some application requirements, it is recommed that return traffic from any of the subnets of R1, should route down the same router through which traffic originated.

NAT as a solution has already been rejected by my Customer, ISP is not ready to change anything on their side, as they have many routers forming that MPLS cloud.

So only config that we change is on these routers.

BGP routers in MPLS cloud would install one route for 192.168.30.x, either through R2 or R3. We cannot do prepending or Med as any client on that LAN need to access both subnets on R1.

Any help on this would be appreciated.

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