A method of optimal routing in an inter-area IGP network based on SR-MPLS Flex-Algorithm procedures, a source node, an area border node and a system thereof are disclosed. The method comprises advertising, by each of the at least two area border nodes of the IGP network comprising a first area and second area, an algorithm-specific metric associated with a destination node from the first set of nodes of the first area to a source node from the second set of nodes of the second area; calculating, by the source node, each inter-area path metric associated with a path including a respective area border node to reach the destination node; and determining, by the source node, an optimal path including one of the at least two area border nodes to reach the destination node based on each of the each calculated inter-area path metric.