Inter-domain routing
Path Computation Elements (PCEs) provide operators of MPLS/GMPLS networks an evolutionary path to Software Defined Networking (SDN) enabling many benefits including flexible/customizable path computation, improved price/performance, and simplified operations. However, in Telco/WAN environments, one of the primary drivers for PCE deployment is seamless inter-domain routing.
For true traffic engineered inter-domain data flow, it must be possible for the control plane to calculate an optimal end-to-end path. That is not possible with the current distributed control plane architecture as no node has access to the full network topology. PCE Servers do have access, via interrogation of other PCE Servers, to the path and traffic engineering information they require to compute an optimal end-to-end path.
For introductory information on PCEs, please see our What is PCE? page.
For more detailed information, see our PCE whitepaper, PCE – an Evolutionary Approach to SDN.
Seamless end-to-end inter-domain routing
A PCE Server gains access to information about another domain by PCE protocol communications with the PCE designated for that domain. As is illustrated below, an interrogated PCE Server provides path information and the relative costs of the possible paths. Although each PCE Server sees only part of the total topology, they use an algorithm called Backwards Recursive Path Computation (BRPC), defined in RFC 5441, to ensure that the calculated path is fully optimized end to-end.
In the diagram, node C, in domain A, sends a path request to PCE Server A for a path to a node Q, in domain B. PCE Server A sends a request for information to PCE Server B in domain B. PCE Server B sends back path options with relative costs. PCE Server A can then compute a full path through domains A and B, taking into account the cost of paths in domain A which it knows from its own topology database, and the additional cost of the path from X to Q and from Y to Q. PCE Server A provides node C with the optimal path.
PCE Servers communicate information to allow path computation across multiple network domains
Related links: