Hello Aysar
Your observation is absolutely correct for the lab scenario, and thank you for sharing your results with us, it’s always helpful! By swapping the MED values, you can achieve symmetric routing where traffic flows through R2/R4 in both directions without using AIGP. This is a valid solution for this specific case.
However, in the lesson, an attempt was made to manually tune the MED values of each AS to match the IGP costs internally. Specifically, the MED values were set to reflect the internal IGP cost to the destination, so the other AS chooses the closest egress.
That is why the MED values were assigned as they were. You may have been able to achieve symmetric routing by swapping them, however, the closest egress is no longer chosen, resulting in inefficiencies due to the internal IGP metrics, and that is still a problem.
AIGP allows the accumulated IGP metric to flow across AS boundaries, so both ASes compute path preference using the same metric. That way, you get symmetric routing, AND you achieve the proper path selection. Does that make sense?
I hope this has been helpful!
Laz