10.6. Source Routing

Routing of an optical layer connection requires that the entire path for connection be computed at the ingress node and signaled to other nodes in the path. In order to accomplish this, the ingress node computes an explicit route, also known as a source route, from the ingress to the egress that includes all the nodes on the path. During path setup, the entire route is carried by the signaling protocol from the ingress to the egress node as the path is established. The intermediate nodes on that route do not perform routing computations, but merely use the explicit route information to forward the setup message (see Chapter 7). Since the ingress node computes the entire route, the possibility of a routing loop is eliminated. Consequently, the restriction that all nodes should run the same routing algorithm can also be relaxed. But as noted in the previous sections, the routing algorithm must take into account issues such as computing physically diverse primary and backup paths, additional constraints on switching capabilities, optical domain constraints, and so on.