RFC 3031

Network Working Group E. Rosen Request for Comments: 3031 Cisco Systems, Inc. Category: Standards Track A. Viswanathan Force10 Networks, Inc. R. Callon Juniper Networks, Inc. January 2001

           Multiprotocol Label Switching Architecture

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document specifies the architecture for Multiprotocol Label Switching (MPLS).

Table of Contents

1 Specification ...................................... 3 2 Introduction to MPLS ............................... 3 2.1 Overview ........................................... 4 2.2 Terminology ........................................ 6 2.3 Acronyms and Abbreviations ......................... 9 2.4 Acknowledgments .................................... 9 3 MPLS Basics ........................................ 9 3.1 Labels ............................................. 9 3.2 Upstream and Downstream LSRs ....................... 10 3.3 Labeled Packet ..................................... 11 3.4 Label Assignment and Distribution .................. 11 3.5 Attributes of a Label Binding ...................... 11 3.6 Label Distribution Protocols ....................... 11 3.7 Unsolicited Downstream vs. Downstream-on-Demand .... 12 3.8 Label Retention Mode ............................... 12 3.9 The Label Stack .................................... 13 3.10 The Next Hop Label Forwarding Entry (NHLFE) ........ 13 3.11 Incoming Label Map (ILM) ........................... 14

3.12 FEC-to-NHLFE Map (FTN) ............................. 14 3.13 Label Swapping ..................................... 15 3.14 Scope and Uniqueness of Labels ..................... 15 3.15 Label Switched Path (LSP), LSP Ingress, LSP Egress . 16 3.16 Penultimate Hop Popping ............................ 18 3.17 LSP Next Hop ....................................... 20 3.18 Invalid Incoming Labels ............................ 20 3.19 LSP Control: Ordered versus Independent ............ 20 3.20 Aggregation ........................................ 21 3.21 Route Selection .................................... 23 3.22 Lack of Outgoing Label ............................. 24 3.23 Time-to-Live (TTL) ................................. 24 3.24 Loop Control ....................................... 25 3.25 Label Encodings .................................... 26 3.25.1 MPLS-specific Hardware and/or Software ............. 26 3.25.2 ATM Switches as LSRs ............................... 26 3.25.3 Interoperability among Encoding Techniques ......... 28 3.26 Label Merging ...................................... 28 3.26.1 Non-merging LSRs ................................... 29 3.26.2 Labels for Merging and Non-Merging LSRs ............ 30 3.26.3 Merge over ATM ..................................... 31 3.26.3.1 Methods of Eliminating Cell Interleave ............. 31 3.26.3.2 Interoperation: VC Merge, VP Merge, and Non-Merge .. 31 3.27 Tunnels and Hierarchy .............................. 32 3.27.1 Hop-by-Hop Routed Tunnel ........................... 32 3.27.2 Explicitly Routed Tunnel ........................... 33 3.27.3 LSP Tunnels ........................................ 33 3.27.4 Hierarchy: LSP Tunnels within LSPs ................. 33 3.27.5 Label Distribution Peering and Hierarchy ........... 34 3.28 Label Distribution Protocol Transport .............. 35 3.29 Why More than one Label Distribution Protocol? ..... 36 3.29.1 BGP and LDP ........................................ 36 3.29.2 Labels for RSVP Flowspecs .......................... 36 3.29.3 Labels for Explicitly Routed LSPs .................. 36 3.30 Multicast .......................................... 37 4 Some Applications of MPLS .......................... 37 4.1 MPLS and Hop by Hop Routed Traffic ................. 37 4.1.1 Labels for Address Prefixes ........................ 37 4.1.2 Distributing Labels for Address Prefixes ........... 37 4.1.2.1 Label Distribution Peers for an Address Prefix ..... 37 4.1.2.2 Distributing Labels ................................ 38 4.1.3 Using the Hop by Hop path as the LSP ............... 39 4.1.4 LSP Egress and LSP Proxy Egress .................... 39 4.1.5 The Implicit NULL Label ............................ 40 4.1.6 Option: Egress-Targeted Label Assignment ........... 40 4.2 MPLS and Explicitly Routed LSPs .................... 42 4.2.1 Explicitly Routed LSP Tunnels ...................... 42 4.3 Label Stacks and Implicit Peering .................. 43

4.4 MPLS and Multi-Path Routing ........................ 44 4.5 LSP Trees as Multipoint-to-Point Entities .......... 44 4.6 LSP Tunneling between BGP Border Routers ........... 45 4.7 Other Uses of Hop-by-Hop Routed LSP Tunnels ........ 47 4.8 MPLS and Multicast ................................. 47 5 Label Distribution Procedures (Hop-by-Hop) ......... 47 5.1 The Procedures for Advertising and Using labels .... 48 5.1.1 Downstream LSR: Distribution Procedure ............. 48 5.1.1.1 PushUnconditional .................................. 49 5.1.1.2 PushConditional .................................... 49 5.1.1.3 PulledUnconditional ................................ 49 5.1.1.4 PulledConditional .................................. 50 5.1.2 Upstream LSR: Request Procedure .................... 51 5.1.2.1 RequestNever ....................................... 51 5.1.2.2 RequestWhenNeeded .................................. 51 5.1.2.3 RequestOnRequest ................................... 51 5.1.3 Upstream LSR: NotAvailable Procedure ............... 52 5.1.3.1 RequestRetry ....................................... 52 5.1.3.2 RequestNoRetry ..................................... 52 5.1.4 Upstream LSR: Release Procedure .................... 52 5.1.4.1 ReleaseOnChange .................................... 52 5.1.4.2 NoReleaseOnChange .................................. 53 5.1.5 Upstream LSR: labelUse Procedure ................... 53 5.1.5.1 UseImmediate ....................................... 53 5.1.5.2 UseIfLoopNotDetected ............................... 53 5.1.6 Downstream LSR: Withdraw Procedure ................. 53 5.2 MPLS Schemes: Supported Combinations of Procedures . 54 5.2.1 Schemes for LSRs that Support Label Merging ........ 55 5.2.2 Schemes for LSRs that do not Support Label Merging . 56 5.2.3 Interoperability Considerations .................... 57 6 Security Considerations ............................ 58 7 Intellectual Property .............................. 58 8 Authors' Addresses ................................. 59 9 References ......................................... 59 10 Full Copyright Statement ........................... 61

Table of Contents​

Table of Contents