RFC 3785 (rfc3785) - Page 2 of 8
Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric
Alternative Format: Original Text Document
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004 These IGP routing protocol extensions currently include advertisement of a single additional MPLS TE metric to be used for Constraint Based Routing of TE tunnels. However, the objective of traffic engineering is to optimize the use and the performance of the network. So it seems relevant that TE tunnel placement may be optimized according to different optimization criteria. For example, some Service Providers want to perform traffic engineering of different classes of service separately so that each class of Service is transported on a different TE tunnel. One example motivation for doing so is to apply different fast restoration policies to the different classes of service. Another example motivation is to take advantage of separate Constraint Based Routing in order to meet the different Quality of Service (QoS) objectives of each Class of Service. Depending on QoS objectives one may require either (a) enforcement by Constraint Based Routing of different bandwidth constraints for the different classes of service as defined in [DS-TE], or (b) optimizing on a different metric during Constraint Based Routing or (c) both. This document discusses how optimizing on a different metric can be achieved during Constraint Based Routing. The most common scenario for a different metric calls for optimization of a metric reflecting delay (mainly propagation delay) when Constraint Based Routing TE Label Switched Paths (LSPs) that will be transporting voice, while optimizing a more usual metric (e.g., reflecting link bandwidth) when Constraint Based Routing TE LSPs that will be transporting data. Additional IGP protocol extensions could be defined so that multiple TE metrics could be advertised in the IGP (as proposed for example in [METRICS]) and would thus be available to Constraint Based Routing in order to optimize on a different metric. However this document describes how optimizing on a different metric can be achieved today by existing implementations and deployments, without any additional IGP extensions beyond [ISIS-TE] and [OSPF-TE], by effectively using the IGP metric as a "second" TE metric. 2. Common Practice In current MPLS TE deployments, network administrators often want Constraint Based Routing of TE LSPs carrying data traffic to be based on the same metric as the metric used for Shortest Path Routing. Where this is the case, this practice allows the Constraint Based Routing algorithm running on the Head-End LSR to use the IGP metric advertised in the IGP to compute paths for data TE LSPs instead of the advertised TE metric. The TE metric can then be used to convey Le Faucheur, et al. Best Current Practice
