Segment Routing MPLS Interworking with LDP
RFC 8661

Document Type RFC - Proposed Standard (December 2019; No errata)
Authors Ahmed Bashandy  , Clarence Filsfils  , Stefano Previdi  , Bruno Decraene  , Stephane Litkowski 
Last updated 2019-12-06
Replaces draft-filsfils-spring-segment-routing-ldp-interop
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Rob Shakir
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IESG IESG state RFC 8661 (Proposed Standard)
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Responsible AD Alvaro Retana
Send notices to, Rob Shakir <>
IANA IANA review state Version Changed - Review Needed
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Internet Engineering Task Force (IETF)                  A. Bashandy, Ed.
Request for Comments: 8661                                    Individual
Category: Standards Track                               C. Filsfils, Ed.
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                              S. Previdi
                                                     Huawei Technologies
                                                             B. Decraene
                                                            S. Litkowski
                                                           December 2019

               Segment Routing MPLS Interworking with LDP


   A Segment Routing (SR) node steers a packet through a controlled set
   of instructions, called segments, by prepending the packet with an SR
   header.  A segment can represent any instruction, topological or
   service based.  SR allows enforcing a flow through any topological
   path while maintaining per-flow state only at the ingress node to the
   SR domain.

   The Segment Routing architecture can be directly applied to the MPLS
   data plane with no change in the forwarding plane.  This document
   describes how Segment Routing MPLS operates in a network where LDP is
   deployed and in the case where SR-capable and non-SR-capable nodes

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  SR-LDP Ships-in-the-Night Coexistence
     2.1.  MPLS2MPLS, MPLS2IP, and IP2MPLS Coexistence
   3.  SR and LDP Interworking
     3.1.  LDP to SR
       3.1.1.  LDP to SR Behavior
     3.2.  SR to LDP
       3.2.1.  Segment Routing Mapping Server (SRMS)
       3.2.2.  SR to LDP Behavior
       3.2.3.  Interoperability of Multiple SRMSes and Prefix-SID
   4.  SR-LDP Interworking Use Cases
     4.1.  SR Protection of LDP-Based Traffic
     4.2.  Eliminating Targeted LDP Sessions
     4.3.  Guaranteed FRR Coverage
     4.4.  Inter-AS Option C, Carrier's Carrier
   5.  IANA Considerations
   6.  Manageability Considerations
     6.1.  SR and LDP Coexistence
     6.2.  Data-Plane Verification
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Migration from LDP to SR
   Authors' Addresses

1.  Introduction

   Segment Routing, as described in [RFC8402], can be used on top of the
   MPLS data plane without any modification as described in [RFC8660].

   Segment Routing control plane can coexist with current label
   distribution protocols such as LDP [RFC5036].

   This document outlines the mechanisms through which SR interworks
   with LDP in cases where a mix of SR-capable and non-SR-capable
   routers coexist within the same network and more precisely in the
   same routing domain.

   Section 2 describes the coexistence of SR with other MPLS control-
   plane protocols.  Section 3 documents the interworking between SR and
   LDP in the case of nonhomogeneous deployment.  Section 4 describes
   how a partial SR deployment can be used to provide SR benefits to
   LDP-based traffic including a possible application of SR in the
   context of interdomain MPLS use cases.  Appendix A documents a method
   to migrate from LDP to SR-based MPLS tunneling.

   Typically, an implementation will allow an operator to select
   (through configuration) which of the described modes of SR and LDP
   coexistence to use.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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