Segment Routing in MPLS Networks E-Book

Hemant Sharma

• Herausgeber: Packt Publishing
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 352

Veröffentlichungsjahr: 2024

Beschreibung

In this book, you will learn about segment routing (SR), a cornerstone of modern networking, and topology-independent loop-free alternate (TI-LFA), with hands-on labs to hit the ground running. Written by a key contributor to global segment routing multiprotocol label switching (SR-MPLS) network designs, this book makes learning SR-MPLS both accessible and insightful.
The book offers an extensive learning path, taking you from a traditional MPLS network using the label distribution protocol (LDP) to a modern SR-MPLS network using SR, and provides a holistic view of their interworking. You’ll also explore TI-LFA fast reroute protection scenarios, and find out how SR-MPLS strengthens network reliability with its source routing paradigm. Using intermediate system–to–intermediate system (IS-IS) as the foundational link-state routing protocol, the chapters ensure that you get a clear understanding of both SR and IS-IS. Following a consistent network topology throughout, this book enables smooth transitions between scenarios, helping you master the technical concepts without distraction.
By the end of this book, you’ll have a solid grasp of MPLS, SR-MPLS, and TI-LFA fast reroute techniques, ready to tackle real-world networking challenges with confidence.

Leseprobe

Segment Routing in MPLS Networks

Transition from traditional MPLS to SR-MPLS with TI-LFA FRR

Hemant Sharma

Segment Routing in MPLS Networks

Copyright © 2024 Packt Publishing

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.

The author acknowledges the use of cutting-edge AI, such as ChatGPT/Claude/Grammarly, with the sole aim of enhancing the language and clarity within the book, thereby ensuring a smooth reading experience for readers. It’s important to note that the content itself has been crafted by the author and edited by a professional publishing team.

Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the author, nor Packt Publishing or its dealers and distributors, will be held liable for any damages caused or alleged to have been caused directly or indirectly by this book.

Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.

Group Product Manager: Dhruv J. Kataria

Publishing Product Manager: Prachi Sawant

Book Project Manager: Uma Devi Lakshmikanth

Senior Editor: Roshan Ravi Kumar

Technical Editor: Arjun Varma

Proofreader: Roshan Ravi Kumar

Copy Editor: Safis Editing

Indexer: Rekha Nair

Production Designer: Alishon Mendonca

DevRel Marketing Coordinator: Rohan Dobhal

First published: November 2024

Production reference: 1241024

Published by Packt Publishing Ltd.

Grosvenor House

11 St Paul’s Square

Birmingham

B3 1RB, UK.

ISBN 978-1-83620-321-6

www.packtpub.com

I am sincerely grateful to the individuals and resources that have significantly contributed to my learning and, ultimately, to the creation of this book. Without their invaluable input, this project would not have come to fruition.

I consulted a diverse range of materials throughout the writing process, both online and offline. While not explicitly mentioned here, their collective contributions were indispensable in developing the substance and depth of this work.

To all these sources of knowledge, I offer my deepest thanks for their generosity in sharing their expertise, which has been instrumental in shaping and enriching this book.

– Hemant Sharma

Foreword

If you choose an MPLS-based WAN architecture, Segment Routing MPLS (SR-MPLS) solves many problems and provides several benefits: less control plane overhead, better use of the available link bandwidth, and fast rerouting around failures at a global scale. Sounds almost too good to be true! However, this book can help enterprise and Service Provider (SP) network engineers build trust in SR-MPLS.

That trust begins with a much-experienced author, Hemant Sharma, who fits the model as an expert in the field. I first got to know Hemant via Cisco Champion’s message boards, where his posts revealed his deep skills. Once we got to know each other, I learned more of his career story—an impressive journey. If you ever thought you’d love to have another experienced network engineer beside you to move into a new technology, know that you get that in Hemant and this book. In many ways, this book models what Hemant has seen work with SR-MPLS during his long travels in the SP engineering world.

Hemant’s lab approach to the content made me most excited about the book. He wants you to build confidence and trust with SR-MPLS, and there’s no better way than seeing it work for yourself. The book cycles through new learning followed by labs, with supplied configuration files you can use with EVE-NG and similar tools. Hemant weaves that feature throughout the book.

As for what you should expect to learn, he starts with some MPLS without Segment Routing as a baseline for all. Then, he lays out the core SR-MPLS concepts and features. The rest of the book then works through the variations on the biggest motivation to consider SR-MPLS: the various rerouting options available. If you ever thought about whether SR-MPLS makes sense for your network or how you might implement any of the various rerouting features, this book has you covered!

I expect we’ll all see more from Hemant over the coming years beyond this first published book. He’s active in the Internet Engineering Task Force (IETF)—it was his recent work with BGP Monitoring Protocol (BMP) that first caught my eye. His combination of real-world deep experience and his willingness to create organized and useful content is a wonderful development! I’ll be watching this space!

Wendell Odom

CCIE Enterprise 1624

Author of 10 editions of the CCNA Official Cert Guide from Cisco Press

Contributors

About the author

Hemant Sharma is a seasoned network engineer with over 14 years of experience in SP networks. He holds a Bachelor of Engineering degree in information and technology and is a certified expert with credentials including CCNA, CCNP, and CCIE #28809 (Emeritus). An active contributor to the IETF, Hemant helps shape the future of networking standards. At Vodafone Group, Hemant leads the evolution of their Worldwide MPLS network and was instrumental in establishing the Vodafone Global SR-MPLS Network. He is recognized within Vodafone as an authority on IP/MPLS, MP-BGP, Multicast, and QoS. Hemant remains dedicated to staying updated with the latest networking trends, which underscores his reliability and expertise in the field.

Your exploration of the book is genuinely appreciated, and your feedback holds immense value. Should you identify any errors or areas for improvement, I extend my sincere apologies for any inconvenience caused. Your insights are crucial to the refinement process, and I am fully committed to addressing and enhancing the work accordingly. Your understanding and thoughtful engagement are greatly appreciated.

About the reviewers

Fernando Lopez Pajares is a senior network and security architect with over 16 years of experience in designing sophisticated network and security architectures. He holds a degree in telecommunications engineering and multiple certifications, including the prestigious CCIE (#54222). He has architected large enterprise networks and, at Fortinet, he provided strategic recommendations that guided the optimization of network and security frameworks for his customers. Currently, he is focused on security and is leading the security strategy and architecture at a large enterprise maintaining an interest in network systems, a field about which he is passionate.

Hari Vishnu, a distinguished senior consultant with a CCIE EI certification, is highly regarded for his expertise in designing and implementing data center fabrics utilizing application-centric infrastructure and VXLAN. His skills also extend to designing and commissioning ISP Core, Gateway, and Transport networks.

Shraddha Hegde has 23+ years of experience working for routing protocols in leading network equipment vendors. She is currently working with Juniper Networks. Her areas of expertise include ISIS, OSPF, segment routing, and MPLS technologies. She contributes actively to protocol standardization in IETF and has authored several drafts and RFCs.

Ron Bonica is a distinguished engineer at Juniper Networks, specializing in power management, IPv6, and segment routing. He is active in the IETF, having authored or co-authored 22 RFC documents and served three 2-year terms as co-director of the IETF Operations and Management Area. Ron currently co-chairs the IETF V6OPS Working Group.

Table of Contents

Preface

Part 1 - MPLS Overview and Recap

1

Introduction to Multiprotocol Label Switching (MPLS)

What is MPLS?

Applications of MPLS

Introducing FRR

Introduction of Segment Routing (SR)

What is SR?

Advantages of SR-MPLS

Disadvantages of SR-MPLS

Structure of the book

Network infrastructure

Topology

Summary

References

2

Lab 1 – Getting Started with LDP-Based MPLS Network

Objective

IPv4 address

Template

Configuration

Verification

Interior Gateway Protocol – IS-IS (Intermediate System - Intermediate System)

Template

Configuration

Verification

MPLS LDP

Configuration

Verification

Summary

References

Part 2 - Segment Routing (SR-MPLS)

3

Lab 2 – Introducing Segment Routing MPLS (SR-MPLS)

Objective

Segment Routing MPLS (SR-MPLS)

What is an SID?

Template

Configuration

Verification

Using SR-MPLS in FIB

Configuration

Verification

Summary

References

4

Lab 3 – SR-LDP Interworking

Objective

MPLS domain separation

Configuration

Verification

LDP to SR stitching

Verification

SR to LDP stitching

Segment Routing Mapping Server (SRMS)

Configuration

Verification

Migrating to SR-MPLS

Configuration

Stopping SRMS advertisements

Summary

References

Part 3 - Fast Reroute in SR-MPLS Networks

5

Lab 4 – Introducing TI-LFA (Topology Independent – Loop-Free Alternate)

Objective

Background of fast rerouting

Classic LFA

Remote LFA

Topology Independent Loop-Free Alternate (TI-LFA)

Configuration

Verification

Verifying TI-LFA activation

Verifying backup path availability

Verifying a non-ECMP backup path

Understanding backup path calculation

Primary path

Backup path

Summary

References

6

Lab 5 – Zero-Segment FRR

Objective

Configuration

Verification

Verifying max label support

Verifying the zero-segment backup path

Understanding backup path calculation

Verifying the backup path traceroute

Restoring the topology

Summary

References

7

Lab 6 – Single-Segment FRR

Objective

Configuration

Verification

Verify single-segment backup path

Understanding backup path calculation

Primary path

Backup path

Verifying backup path traceroute

Summary

References

8

Lab 7 – Double-Segment FRR

Objective

Configuration

Verification

Verifying the primary path from P8 to PE5

Verifying double-segment backup path

Understanding backup path calculation

Primary path

Backup path

Verifying the backup path traceroute

Restoring the topology

Summary

References

9

Lab 8 – Microloop Avoidance

Objective

Preparation

Verification

Verify the primary path from P2 to PE5

Microloop avoidance in SR-MPLS networks

Configuration

Trigger microloop avoidance

Verification

Verifying the microloop avoidance primary path from P2 to PE5

Understanding the explicit primary path calculation

Verifying the post-convergence primary path from P2 to PE5

Restore topology

Summary

References

10

Lab 9 – TI-LFA Node Protection

Objective

MPLS TE (MPLS Traffic Engineering)

Enabling MPLS TE on all routers of the topology

Preparing the base topology

Configuration

Verification

Backup path in the RIB

Backup path in the LFIB

Verifying a backup path traceroute

Understanding the backup path calculation

Primary path

Backup path

Restoring the topology

Summary

References

11

Lab 10 – TI-LFA Local SRLG-Disjoint Protection

Objective

Configuration

Verification

Backup path in the RIB

Backup path in the FIB

Backup path in the LFIB

Verifying the backup path traceroute

Understanding the backup path calculation

Primary path

Backup path

Summary

References

12

Lab 11 – TI-LFA Global Weighted SRLG Protection

Objective

Configuration

Verification

Verify P7 advertises SRLG as TLV 238

Backup path in the RIB

Backup path in the FIB

Backup path in the LFIB

Examining the MPLS Traffic-Eng auto-tunnel

Verifying the backup path traceroute

Understanding backup path calculation

Primary path

Backup path

Restoring the topology

Summary

References

13

Lab 12 – TI-LFA Node + SRLG Protection

Objective

Configuration

Verification

Backup path in the RIB

Backup path in the FIB

Backup path in the LFIB

Examining the MPLS traffic-eng auto tunnel

Verifying the backup path traceroute

Understanding the backup path calculation

Primary path

Backup path

Summary

References

14

Lab 13 – TI-LFA Tiebreaker

Preparation

TI-LFA link protection preference

Objective

Configuration

Verification

Understanding backup path preference

TI-LFA SRLG protection preference

Objective

Configuration

Verification

Understanding backup path preference

TI-LFA node protection preference

Objective

Configuration

Verification

Understanding backup path preference

Summary

References

Index

Other Books You May Enjoy

Preface

The journey began with a spark of curiosity that ignited a deep desire to explore the subject firsthand. Eager to grasp the intricacies, I delved into hands-on practice with numerous labs, immersing myself in the practical aspects. As I honed my skills and knowledge, a unique idea emerged – to create a lab guide that could serve as a valuable resource for others venturing into this field.

The vision evolved further, as I realized the potential of consolidating all these efforts into a comprehensive book. Combining theory with practical experience, the book could offer readers a seamless learning path, catering to both their inquisitive minds and their desire for hands-on application.

The journey from curiosity to lab exploration and, finally, to crafting this book has been an exhilarating one. With passion and dedication, I aim to share this wealth of knowledge, hoping to inspire and empower others on their voyage of discovery and growth in this subject.

The primary objective driving the creation of this book is to facilitate a seamless transition from the traditional Multiprotocol Label Switching (MPLS) to the Segment Routing MPLS (SR-MPLS). Recognizing the complexities and challenges that can arise during such a shift, the book endeavors to present the subject matter in a manner that simplifies the process for you.

By offering clear and concise explanations, practical examples, and step-by-step guidance, this book aims to empower network professionals and enthusiasts to adopt SR-MPLS with confidence. It seeks to demystify the intricacies and technicalities associated with the new approach, making it accessible and comprehensible to a wider audience.

The emphasis on simplicity in this book not only serves to ease the transition for experienced network engineers but also extends a welcoming hand to those new to this concept. Through well-structured content and user-friendly language, you are encouraged to embark on this journey with enthusiasm, knowing that you will be equipped with the knowledge and insights necessary to embrace the advancements in networking technology.

Overall, the book’s elaborative approach ensures that you will gain a solid understanding of SR-MPLS, empowering you to implement it effectively and harness its benefits in your networking environments.

Who this book is for

This book is tailored for network professionals operating within MPLS environments, including network engineers, planners, designers, and architects. It particularly caters to those actively involved in SR-MPLS networks or currently in the process of integrating this technology into their network infrastructure.

Structured as a series of labs, the content of this book encompasses both theoretical concepts and practical knowledge. It serves as a comprehensive resource to enhance the understanding and management of SR networks, especially during the transition from traditional MPLS networks.

Upon completing this book, you will have the proficiency to understand, implement, and operate the fundamental elements of the following features within the Cisco IOS-XR network operating system:

IS-IS IGP routing protocolMPLS LDPSR-MPLS in IS-IS networksTI-LFA in SR-MPLS networks

What this book covers

Chapter 1, Introduction to Multiprotocol Label Switching (MPLS), revisits the fundamentals of MPLS, providing you with a concise yet insightful review of its operations. Additionally, it offers valuable perspectives into the basics of segment routing.

Chapter 2, Lab 1 – Getting Started with LDP-Based MPLS Network, walks you through setting up the basic network topology. It covers using IS-IS as the interior gateway routing protocol and LDP as the MPLS forwarding protocol.

Chapter 3, Lab 2 – Introducing Segment Routing MPLS (SR-MPLS), introduces segment routing within an existing MPLS network based on LDP and explains how forwarding operates in both scenarios.

Chapter 4, Lab 3 – SR-LDP Interworking, concentrates on connecting different domains from LDP to SR-MPLS, and vice versa. You will understand how the inter-domain label switch path is created and the essential requirements for it. You will also grasp the significance of SRMS in facilitating interaction between SR-MPLS and traditional MPLS networks.

Chapter 5, Lab 4 – Introducing TI-LFA (Topology Independent – Loop-Free Alternate), introduces TI-LFA in SR-MPLS networks, detailing the calculation and installation of backup paths on routers.

Chapter 6, Lab 5 – Zero-Segment FRR, explores scenarios where backup paths can reroute traffic in the event of network failures, without any additional segments introduced.

Chapter 7, Lab 6 – Single-Segment FRR, examines scenarios where backup paths reroute traffic in case of network failures, requiring only one additional segment for rerouting.

Chapter 8, Lab 7 – Double-Segment FRR, examines scenarios where backup paths reroute traffic in case of network failures, requiring two additional segments.

Chapter 9, Lab 8 – Microloop Avoidance, discusses the occurrence of microloops during network convergence and explores their mitigation within SR-MPLS networks.

Chapter 10, Lab 9 – TI-LFA Node Protection, prepares the network topology for forthcoming TI-LFA scenarios. It specifically delves into the node protection method, where the backup path is computed under the assumption that a link failure implies a node failure.

Chapter 11, Lab 10 – TI-LFA Local SRLG-Disjoint Protection, explores the TI-LFA local-SRLG disjoint scenario, where the backup path is calculated under the assumption that all local SRLG links fail simultaneously.

Chapter 12, Lab 11 – TI-LFA Global Weighted SRLG Protection, builds upon the previous lab, examining scenarios where both local and remote SRLG links are bypassed in the calculation of TI-LFA backup paths.

Chapter 13, Lab 12 – TI-LFA Node + SRLG Protection, delves into a scenario where TI-LFA is tasked with computing and implementing a backup path that circumvents both local as well as remote SRLGs and the involved node.

Chapter 14, Lab 13 – TI-LFA Tiebreaker, addresses scenarios where TI-LFA is tasked with calculating both node protection and SRLG protection simultaneously. It explores how prioritization is determined when both protections can’t be achieved concurrently.

To get the most out of this book

This book tackles a subject that is already extensively covered in articles, books, and videos. However, its unique approach aims to stand out by consolidating all the theoretical knowledge in one comprehensive resource. The central focus is on providing you with a seamless learning experience, supported by practical labs that enable smooth transitions between topics.

By presenting a well-organized compilation of theories and incorporating hands-on lab exercises, this book ensures that you can effortlessly navigate from one concept to another. It caters to both beginners and those with prior knowledge, offering a holistic understanding of the subject matter while facilitating quick and efficient learning.

Whether you are a novice eager to delve into the topic or a seasoned individual seeking to reinforce your expertise, this book serves as a valuable reference, condensing valuable insights from various sources into a single cohesive work.

Within the pages of this book lies a compilation of fundamental building blocks that form the bedrock of segment routing, presented at a rudimentary level. The intentional focus on foundational aspects means that not all the advanced features are covered in this edition.

The decision to limit the scope allows for a clear and coherent presentation of the essential concepts, making it accessible to readers at various stages of familiarity with the subject. However, the author acknowledges that there is still much more to explore and share about segment routing.

To fully leverage the labs featured in this book, it is imperative to have a solid command of the Cisco IOS-XR CLI. This foundational knowledge is complemented by familiarity with IS-IS and LDP running on the Cisco IOS-XR platform, as these technologies serve as the cornerstone for the practical exercises presented here. If you already possess prior experience with these technologies, you will find it more straightforward to follow along and apply the concepts in the labs.

For those who are new to the Cisco IOS-XR CLI, IS-IS, or LDP, there’s no need to be concerned. The book is purposefully structured to provide detailed explanations and support, ensuring you gain the necessary understanding as you progress through the labs. So, whether you are well-versed in these technologies or approaching them for the first time, the book is designed to facilitate a comprehensive learning experience.

This book serves as a valuable resource for gaining foundational knowledge on the subject, but it is not intended to transform you into an expert. Instead, it provides a concise and approachable introduction, equipping you with essential insights and understanding. For those seeking comprehensive expertise, further exploration and additional resources will be necessary. Nevertheless, this book acts as a stepping stone towards a broader comprehension of the subject matter.

Download the example code files

You can download the example code files for this book from GitHub at https://github.com/PacktPublishing/Segment-Routing-in-MPLS-Networks. If there’s an update to the code, it will be updated in the GitHub repository.

We also have other code bundles from our rich catalog of books and videos available at https://github.com/PacktPublishing/. Check them out!

Conventions used

There are a number of text conventions used throughout this book.

Code in text: Indicates code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter/X handles. Here is an example: “The following output from the PE1 router confirms that the IP addresses are applied according to the configurations on each router.”

A block of code is set as follows:

router IS-IS IGP interface GigabitEthernet0/0/0/3 address-family ipv4 unicast metric 100

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

interface {{ interface_name }} description "P(E){{ x }}_to_P(E){{ y}}" ipv4 address {{ xy.0.0.(x or y) }} 255.255.255.0 no shutdown

Any command-line input or output is written as follows:

RP/0/RP0/CPU0:PE1#show isis route detail 5.5.5.5/32 L2 5.5.5.5/32 [40/115] Label: None, medium priority Installed Jan 01 06:02:54.493 for 00:02:56 via 12.0.0.2, GigabitEthernet0/0/0/2, P2, Weight: 0 src PE5.00-00, 5.5.5.5 RP/0/RP0/CPU0:PE1#

Bold: Indicates a new term, an important word, or words that you see onscreen. For instance, words in menus or dialog boxes appear in bold. Here is an example: “Built-in support for SR-Traffic Engineering (SR-TE), allowing operators to direct traffic through specific paths for optimized network performance.”

Tips or important notes

Appear like this.

Get in touch

Feedback from our readers is always welcome.

General feedback: If you have questions about any aspect of this book, email us at [email protected] and mention the book title in the subject of your message.

Errata: Although we have taken every care to ensure the accuracy of our content, mistakes do happen. If you have found a mistake in this book, we would be grateful if you would report this to us. Please visit www.packtpub.com/support/errata and fill in the form.

Piracy: If you come across any illegal copies of our works in any form on the internet, we would be grateful if you would provide us with the location address or website name. Please contact us at [email protected] with a link to the material.

If you are interested in becoming an author: If there is a topic that you have expertise in and you are interested in either writing or contributing to a book, please visit authors.packtpub.com.

Share Your Thoughts

Once you’ve read Segment Routing in MPLS Networks, we’d love to hear your thoughts! Please click here to go straight to the Amazon review page for this book and share your feedback.

Your review is important to us and the tech community and will help us make sure we’re delivering excellent quality content.

Share Your Thoughts

Once you’ve read Segment Routing in MPLS Networks, we’d love to hear your thoughts! Please click here to go straight to the Amazon review page for this book and share your feedback.

Your review is important to us and the tech community and will help us make sure we’re delivering excellent quality content.

Download a free PDF copy of this book

Thanks for purchasing this book!

Do you like to read on the go but are unable to carry your print books everywhere?

Is your eBook purchase not compatible with the device of your choice?

Don’t worry, now with every Packt book you get a DRM-free PDF version of that book at no cost.

Read anywhere, any place, on any device. Search, copy, and paste code from your favorite technical books directly into your application.

The perks don’t stop there, you can get exclusive access to discounts, newsletters, and great free content in your inbox daily

Follow these simple steps to get the benefits:

Scan the QR code or visit the link below

https://packt.link/free-ebook/978-1-83620-321-6

Submit your proof of purchaseThat’s it! We’ll send your free PDF and other benefits to your email directly

Part 1 - MPLS Overview and Recap

In this part, you are provided with a comprehensive understanding of the workings of traditional MPLS networks. It incorporates background information on SR-MPLS and fast rerouting in MPLS. Additionally, the part delves into the network infrastructure, outlining the topology specifically designed for the book’s context and elaborating on its fundamental building blocks.

This part contains the following chapters:

Chapter 1, Introduction to Multiprotocol Label Switching (MPLS)Chapter 2, Lab 1 – Getting Started with LDP-Based MPLS Network

1

Introduction to Multiprotocol Label Switching (MPLS)

This chapter aims to explain Multiprotocol Label Switching (MPLS), an important technology in Transmission Control Protocol (TCP) / Internet Protocol (IP) networks, and its continued relevance. It also introduces Fast Reroute (FRR) in MPLS networks and Segment Routing MPLS (SR-MPLS), a newer method for operating MPLS networks. Before beginning the practical exercises, it explains the structure, the key components needed to build the required infrastructure, and where to obtain them. Throughout, you are encouraged to revise the fundamental concepts of MPLS, SR-MPLS, and FRR. Understanding these concepts is crucial for successfully following the practical exercises and gaining deeper insight into advanced networking principles in later chapters. By the end of this chapter, you will have understood MPLS concepts, their applications, and the required infrastructure for hands-on experience.

This chapter will cover the following topics:

What is MPLS?Applications of MPLSIntroducing FRRIntroduction of segment routing (SR)Structure of the bookNetwork infrastructure

Let us dive in!

What is MPLS?

In the TCP/IP model, the Internet Layer (Layer 3) is responsible for managing IP addressing and routing packets between different networks. MPLS sits between the traditional Layer 2 (Network Access Layer) and Layer 3 (Internet Layer) in the TCP/IP model, which is sometimes referred to as Layer 2.5 or Shim Header.

Figure 1.1 – MPLS Header

The MPLS header comprises four fields, totaling 32 bits in size:

Label: The Label field is the most critical part of the MPLS header, comprising a fixed size of 20 bits that carries the forwarding information for the packet. Each router in an MPLS-enabled network maintains a forwarding table that maps incoming labels to the next-hop router and the corresponding output interface. The label is used to quickly determine the packet’s path through the network, making the forwarding process more efficient.Experimental (EXP) or Traffic Class (TC) bits: The Experimental bits field is 3 bits long and is commonly used for implementing QoS in MPLS networks. By setting different values in the EXP/TC field, certain types of traffic can be prioritized over others, helping to ensure that critical or high-priority data receives preferential treatment in the network.S (bottom of stack) bit: This is a single bit that indicates whether the current label is the last one in the MPLS stack. MPLS supports label stacking, which means multiple MPLS labels can be used hierarchically. When the S bit is set to 1, it means this is the last label in the stack.Time to live (TTL): The TTL field in the MPLS header functions similarly to the TTL in IP headers. It is an 8-bit field that is decremented at each MPLS hop. If the TTL reaches zero, the packet is discarded. This helps prevent packets from being trapped in routing loops.

The labs in this hands-on guide will focus on exploring MPLS labels, the label stack, and TTL (through the MPLS path) across various outputs. However, the EXP bit falls outside the scope of this book and will not be covered.

MPLS is a technology that transforms traditional TCP/IP networks through the introduction of a label-switching mechanism. This approach significantly improves the efficiency and scalability of network routing. Here’s an overview of how MPLS changes TCP/IP networks with its label-switching mechanism:

Label allocation: A label is a short, fixed-length, locally significant identifier that is associated with a forwarding equivalence class (FEC), which represents a group of packets that are treated and forwarded in the same way through the network. The FEC groups packets that share the same forwarding path through the network, often based on factors such as their destination IP address, but the label itself does not represent or encode this address. Instead, the label is simply used to direct packets along the pre-determined path without referencing the full destination IP address at each routing step.Label distribution: MPLS routers use label distribution protocols, such as Label Distribution Protocol (LDP), Resource Reservation Protocol with Traffic Engineering (RSVP-TE), or Border Gateway Protocol (BGP), to exchange label information among themselves. Labels are distributed throughout the MPLS network, and routers build forwarding tables based on this information.Label switching: Instead of traditional IP routing, where routers make forwarding decisions based on the destination IP address, MPLS routers make forwarding decisions based on the MPLS label. When a packet enters the MPLS network, the ingress router attaches a label to the packet based on its FEC. As the packet traverses the network, intermediate routers forward it by performing label operations—namely, push, swap, and pop.Push: When a packet enters the MPLS network, a label may be added (or “pushed”) onto the packet.Swap: When the packet reaches an intermediate router, the existing label is replaced (or “swapped”) with a new one, which determines the next hop in the routing process.

This operation is necessary because a label is a local construct on each router; they are dynamically allocated and not globally unique in traditional MPLS label distribution methods. Consequently, labels contain locally significant information for the routers. During the swap process, the router replaces the incoming label, which it had locally allocated and distributed, with the outgoing label received from the next-hop router along the path, ensuring efficient and accurate packet forwarding.

Pop: Finally, when the packet exits the MPLS network at the egress router, the label is removed (or “popped”), allowing the packet to be forwarded based on its original IP header.

Additionally, Penultimate Hop Popping (PHP) may occur, where the penultimate router (the one just before the egress router) removes the label instead. This approach reduces processing at the egress router, allowing it to forward the packet directly using the original IP header, thereby enhancing overall network efficiency.

This label-based approach allows MPLS routers to make efficient forwarding decisions without needing to examine the IP header.

Label stack: MPLS allows for the stacking of labels, forming a label stack. This is useful for scenarios such as MPLS virtual private networks (VPNs) and traffic engineering. Each label in the stack represents a different forwarding decision along the packet’s path through the network. The top label, referred to as the transport label, is responsible for forwarding decisions within the MPLS network. In a multi-domain MPLS environment, multiple labels in the stack may serve as transport labels across different domain boundaries. Beneath these transport labels, there can be service labels, which represent L2-VPN or L3-VPN services. These service labels remain untouched throughout the packet’s transit within the MPLS core and are only processed by the edge routers.Traffic engineering: MPLS supports traffic engineering, allowing network operators to optimize the use of network resources and control the flow of traffic through the network. By manipulating the labels and paths, operators can achieve better load balancing and resource utilization. It provides operators with greater flexibility to address the requirements of various use cases effectively.Service agnostic operation: One of the key benefits of MPLS features a BGP-free core, which reduces complexity by avoiding the need for core routers to carry extensive BGP routing tables. Additionally, mid-point core routers in MPLS networks operate without detailed service-specific forwarding information, showcasing inherent flexibility.VPNs: MPLS is commonly used for building Layer 2 and Layer 3 VPNs. Labels are used to distinguish between different VPNs, enabling the coexistence of multiple virtual networks over a shared physical infrastructure.

In essence, MPLS operates within the TCP/IP model, nestled between the traditional Layer 2 and Layer 3. This strategic placement allows MPLS to streamline packet forwarding through the network by utilizing a specialized header structure and MPLS operations.

Applications of MPLS

MPLS has found widespread adoption across various sectors and network types. It is frequently used in the following:

Internet service providers (ISPs): MPLS is essential for ISPs, enabling flexible, scalable services with multi-protocol capabilities and efficient traffic engineering, ensuring optimal network performance for customers.VPNs: MPLS-based VPNs provide secure, isolated communication channels over shared network infrastructure, offering businesses a cost-effective and secure way to interconnect their geographically dispersed offices.Mobile networks: The utilization of MPLS in mobile backhaul networks is on the rise, effectively managing the surge in traffic from the increasing number of mobile devices. This ensures a secure and smooth user experience, with the ability to separate internet, voice, signaling, and management traffic over a shared infrastructure.Enterprise networks: Enterprises benefit from MPLS by establishing secure and streamlined wide area networks (WANs) for various services. This enables reliable, high-speed data transmission across multiple locations, ensuring scalability, flexibility, and optimized traffic engineering for improved network performance.

MPLS is a versatile solution across diverse sectors and network landscapes, playing pivotal roles in enterprise networks, ISPs, VPNs, and mobile networks, each benefiting from its unique capabilities and advantages.

With a thorough understanding of MPLS and its benefits in enhancing network efficiency and performance, the focus now shifts to FRR, which serves as a critical mechanism within MPLS networks, providing rapid failure recovery to ensure minimal disruption in data transmission. This capability is crucial for maintaining high availability and reliability in modern networks. The upcoming section will delve into the principles of FRR.

Introducing FRR

FRR is a mechanism used in IP/MPLS networks to minimize the impact of network failures on the traffic flow. When a fault occurs in the network, the response varies depending on the protocol in use. Generally, there are two primary approaches to address the issue:

Wait for convergence: In both traditional MPLS protocols, LDP and RSVP-TE, the network initiates a reconvergence process to identify the optimal new path from the source to the destination. The MPLS label distribution protocols rely on this reconvergence to re-establish the end-to-end MPLS path over the newly identified route. This period is known as convergence delay, during which traffic may be dropped, leading to the undesirable phenomenon of traffic black holes.Local fast rerouting at the Point of Local Repair (PLR): While the network is reconverging, the router that detects the fault can reroute traffic around the failure, ensuring continuity of service. This diversion typically occurs within approximately 50 milliseconds, utilizing a precomputed backup path available on the PLR, referred to as the FRR path. Typically, all routers in the network are configured to support FRR, and the designation of the PLR may shift among routers based on the location of each fault. The FRR backup path remains active and carries traffic until the convergence process is complete and the new primary path is integrated into the routing and forwarding tables of the routers.

Although RSVP-TE protocols are outside the scope of this book, the LDP protocol is discussed in the next chapter, focusing primarily on MPLS forwarding rather than on failure and FRR mechanisms. The FRR method utilizing SR in this book is called Topology Independent Loop-Free Alternate (TI-LFA) and will be explored in greater detail in later chapters.

By promptly diverting traffic along pre-established alternate paths, FRR aims to minimize downtime and packet loss. This proactive approach ensures a resilient network infrastructure, allowing data packets to seamlessly reach their destinations despite potential disruptions in the network.

Here’s a general understanding of how FRR works:

Protection paths: FRR involves precomputing backup or protection paths that can quickly take over in the event of a failure. These paths are computed in advance and installed in the forwarding database for prompt activation when necessary.Node or link failure detection: When a failure occurs, such as a link or a node going down, the network devices detect the failure. This detection can be done through various mechanisms such as monitoring link status, using routing protocols, or employing dedicated protocols for fault detection, such as Bidirectional Forwarding Detection (BFD).PLR: FRR commonly utilizes local repair mechanisms, where the affected node, upon detecting a failure, independently decides to reroute traffic through precomputed protection paths. This is a faster alternative to waiting for global network convergence. The node executing local repair is commonly referred to as the PLR.FRR activation: Upon detecting a failure, the network devices (routers or switches) quickly switch traffic to the pre-computed protection paths. This rapid switchover minimizes the impact on ongoing communications and helps maintain network connectivity.

FRR presents a proactive approach to minimize the effects of network failures. This is achieved by preemptively calculating and installing alternative routes for potential points of failure within the network. FRR serves as a robust defense against network downtime and packet loss. By ensuring uninterrupted traffic flow and reducing disruptions to ongoing communications and data in transit, FRR enhances the reliability and resilience of IP/MPLS networks. FRR does come with a cost, as it requires additional processing power and memory to compute and maintain backup path information in an immediately available state.

Upon acquiring a solid understanding of MPLS and establishing foundational knowledge of FRR, the stage is set to introduce SR in the upcoming section. The labs in this book focus on leveraging the best of both worlds, SR-MPLS and TI-LFA FRR. The synergy between the two helps create an enriching learning experience.

Introduction of Segment Routing (SR)

The fundamental distinction between traditional MPLS and SR lies in their simplicity. While traditional MPLS offers benefits such as traffic engineering, Quality of Service (QoS), and support for L2 and L3 VPNs, SR architecture surpasses it. It works on the principle of source routing, where the state lies in the packet and a node routes a packet through an ordered list of instructions called segments embedded in the packet itself. It removes the need for complex signaling protocols and path states in the network.

What is SR?

Segment Routing (SR), or Source Packet Routing in Networking (SPRING), is a routing technology that fundamentally transforms the routing framework, allowing the source node to determine the entire path of a packet, including all intermediate hops, from its origin to the final destination. This predetermined path is expressed as an ordered list of segments, stacked on the packet header. A segment, often identified by its segment identifier (SID), may hold significance either locally to an SR node or globally within an SR domain.

At each hop along the predetermined path, the intermediate node determines its next hop by extracting information encoded in the packet header. This information, originally encoded by the upstream neighbor for the relevant segment, allows intermediate nodes to follow the segment sequence without the need for additional state information. This encoding streamlines the mechanism, reducing the overhead and complexity of traditional IP/MPLS protocols. By adopting SR, the source node gains enhanced control over routing decisions, providing a more adaptable and efficient approach to navigating complex network topologies.

One of the key advantages of SR is that it effectively reduces the network’s statefulness. Since the entire path is encoded in the packet header, the network nodes don’t need to store extensive routing information, as the segment or the list of segments itself carries all the necessary instructions. This stateless nature not only streamlines the network’s operation but also enhances its scalability and resilience. Additionally, the reduced reliance on network-wide state information contributes to faster convergence during link or node failures. With its ability to provide efficient, flexible, and stateless routing, SR has emerged as a promising approach to tackle the challenges posed by modern networking environments.

SR is a flexible networking concept that is not tied to a specific data-plane technology. The architectural principles are detailed in RFC 8402 under the title Segment Routing Architecture. Notably, there are two primary implementations of SR:

SR-MPLS: SR aligns with the original MPLS framework, which did not mandate the use of a specific signaling protocol. Consequently, SR utilizes various control-plane protocols, such as link-state routing protocols, to distribute or advertise segment information known as SIDs. Furthermore, no adjustments are required for the MPLS forwarding plane, as SIDs are encoded as labels, and a list of SIDs is represented as a label stack. These segments, depicted as labels, are distributed through protocols such as Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS) as IGP segments or via BGP as BGP segments.SRv6: The second implementation is SRv6, where SR leverages the IPv6 data plane. However, details about SRv6 are not covered in this particular book, as the IPv6 data plane operates quite differently from the MPLS data plane.

Note

The book exclusively focuses on SR-MPLS and its transition from the LDP, with IS-IS chosen as the Interior Gateway Protocol (IGP) for SR-MPLS label distribution. In the context of this book, the terms SR-MPLS and SR-IS-IS are used interchangeably, highlighting IS-IS as the selected IGP for establishing the SR-MPLS network.

MPLS has been a long-standing technology in networking, continuously evolving with the addition of various features over the years. One of the challenges with maintaining label integrity in MPLS lies in the traditional label distribution protocols, such as LDP and RSVP-TE. These protocols necessitate nodes to create and maintain sessions or states, leading to signaling message overheads that may demand extra computational power on routers. Additionally, maintaining signaling sessions and state information on routers can pose scalability challenges.

However, the next phase in its evolution is SR-MPLS.

Advantages of SR-MPLS

SR addresses several key problems in traditional MPLS networking, making it a powerful and flexible solution for various use cases.

Some of the significant problems that SR helps to solve include the following:

Label distribution: Traditional MPLS uses LDP, RSVP-TE, or BGP for distributing labels whereas SR introduces label distribution through only IGP and BGP, allowing for greater flexibility and simplified label management.Stateless operation: LDP and RSVP-TE require routers to maintain state information about label bindings, leading to increased protocol complexity and memory usage. SR-MPLS operates in a stateless manner. It relies on source-based forwarding instructions encoded in the MPLS header, simplifying the control plane, ensuring efficient scalability, and providing flexibility for network changes.Traffic engineering: Built-in support for SR-Traffic Engineering (SR-TE), allowing operators to direct traffic through specific paths for optimized network performance.

Transitioning from traditional MPLS to SR-MPLS is a smooth process without any challenges, as described in the following steps:

Seamless network migration