A method for routing and forwarding data packets to a legacy IP network, which uses a next-hop routing protocol, through a source-selected path routing network comprising a plurality of autonomous systems, ASes (131, 132, 133, 134, 135), wherein data packets are transmitted from a node A1 of the legacy IP network A to a node B1 of a legacy IP network B, wherein the method comprises the steps of: announcing, by the node B1 of the legacy IP network B, routing information to a first node within the source-selected path routing network using an exterior gateway protocol, wherein said source-selected path routing network is a direct neighbor of the legacy IP network B; announcing the routing information announced by the node B1 to a second node within the source-selected path routing network using an internal source-selected path routing network protocol; announcing, by the second node to the node A1 of the legacy IP network A being a direct neighbor of the source-selected path routing network, the routing information announced to the first node by the node B1 without announcing routing information necessary for routing data packets within the source-selected path routing network between the first and second node, wherein the source-selected path routing network is configured to act as a single backbone AS when communicating with the node A1 of the legacy IP network A using an exterior gateway protocol; forwarding data packets from the node A1 destined for the node B1 to the second node using the next-hop routing protocol; forwarding the data packets from the second node to the first node along a selected path via a plurality of autonomous systems (131, 132, 133, 134, 135) of the source-selected path routing network; and forwarding the data packets from the first node to the node B1 using the next-hop routing protocol. announcing, by the node B1 of the legacy IP network B, routing information to a first node within the source-selected path routing network using an exterior gateway protocol, wherein said source-selected path routing network is a direct neighbor of the legacy IP network B; announcing the routing information announced by the node B1 to a second node within the source-selected path routing network using an internal source-selected path routing network protocol; announcing, by the second node to the node A1 of the legacy IP network A being a direct neighbor of the source-selected path routing network, the routing information announced to the first node by the node B1 without announcing routing information necessary for routing data packets within the source-selected path routing network between the first and second node, wherein the source-selected path routing network is configured to act as a single backbone AS when communicating with the node A1 of the legacy IP network A using an exterior gateway protocol; forwarding data packets from the node A1 destined for the node B1 to the second node using the next-hop routing protocol; forwarding the data packets from the second node to the first node along a selected path via a plurality of autonomous systems (131, 132, 133, 134, 135) of the source-selected path routing network; and forwarding the data packets from the first node to the node B1 using the next-hop routing protocol.