Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric
RFC 3785

Document Type RFC - Best Current Practice (May 2004; No errata)
Also known as BCP 87
Last updated 2015-10-14
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IESG IESG state RFC 3785 (Best Current Practice)
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Network Working Group                                     F. Le Faucheur
Request for Comments: 3785                                     R. Uppili
BCP: 87                                              Cisco Systems, Inc.
Category: Best Current Practice                              A. Vedrenne
                                                               P. Merckx
                                                                  Equant
                                                              T. Telkamp
                                                         Global Crossing
                                                                May 2004

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

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document describes a common practice on how the existing metric
   of Interior Gateway Protocols (IGP) can be used as an alternative
   metric to the Traffic Engineering (TE) metric for Constraint Based
   Routing of MultiProtocol Label Switching (MPLS) Traffic Engineering
   tunnels.  This effectively results in the ability to perform
   Constraint Based Routing with optimization of one metric (e.g., link
   bandwidth) for some Traffic Engineering tunnels (e.g., Data Trunks)
   while optimizing another metric (e.g., propagation delay) for some
   other tunnels with different requirements (e.g., Voice Trunks).  No
   protocol extensions or modifications are required.  This text
   documents current router implementations and deployment practices.

1.  Introduction

   Interior Gateway Protocol (IGP) routing protocols (OSPF and IS-IS) as
   well as MultiProtocol Label Switching (MPLS) signaling protocols
   (RSVP-TE and CR-LDP) have been extended (as specified in [ISIS-TE],
   [OSPF-TE], [RSVP-TE] and [CR-LDP]) in order to support the Traffic
   Engineering (TE) functionality as defined in [TE-REQ].

Le Faucheur, et al.      Best Current Practice                  [Page 1]
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
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