RFC 3785 (rfc3785) - Page 2 of 8

Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric

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