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Welcome to Day One


This book is part of the Day One library, produced and published by Juniper Networks Books.

Day One books cover the Junos OS and Juniper Networks networking essentials with straightforward explanations, step-by-step instructions, and practical examples that are easy to follow. You can obtain the books from various sources:

  • Download a free PDF edition at

  • Many of the library’s books are available on the Juniper app: Junos Genius.

  • Get the ebook edition for iPhones and iPads from the iBooks Store. Search for Juniper Networks Books or the title of this book.

  • Get the ebook edition for any device that runs the Kindle app (Android, Kindle, iPad, PC, or Mac) by opening your device’s Kindle app and going to the Amazon Kindle Store. Search for Juniper Networks Books or the title of this book.

  • Purchase the paper edition at Vervante Corporation ( for between $15-$40, depending on page length.

  • Note that most mobile devices can also view PDF files.

Key Segment Routing Resources

Segment routing (SR) is a strategic end-to-end transport architecture. There are some key resources you should be familiar with before starting this book:

What You Need to Know Before Reading This Book

The authors have made a few assumptions about the general knowledge level of the reader:

  • You have read Day One: Configuring Segment Routing with Junos as mentioned on the previous page.

  • You understand how existing Multiprotocol Label Switching (MPLS) networks are designed, configured, and how they function. This includes a basic understanding of IGPs, BGP, LDP, and RSVP-TE.

  • You understand how traffic engineering improves resource utilization in current wide-area networks.


There are several books in the Day One library covering MPLS, BGP, and the Junos CLI for various levels of expertise:

What You Will Learn by Reading This Book

This book will help you to:

  • Successfully understand and implement solutions to the variety of problems segment routing (SR) solves: both IPv4 and native IPv6 connectivity, traffic protection, traffic engineering for constraints- based routing, and bandwidth optimization.

  • Plan controlled SR deployments in brownfield networks and migrate away from LDP and/or RSVP-TE. This book accretes SR functionality to a typical service provider wide-area network, gradually replac- ing the existing with equivalent, and in most cases superior, functionality.

  • Appreciate the importance of modern telemetry stacks, how they improve upon legacy protocols such as SNMP, and their relevance to how bandwidth optimization can be performed.

  • Understand the role centralized traffic controllers can play in complex networks, with their ability to visualize the topology, as well as direct both the underlay and overlay.

  • Explore cutting-edge tools such as SR-over-UDP and FlexAlgo. SRoUDP is gaining relevance as a viable data plane for non-SR-MPLS capable platforms. FlexAlgo is a fresh look at multi-topology routing, without the limitations of earlier approaches.

Glossary of Used Terms


Border Gateway Protocol


Border Gateway Protocol Labeled Unicast


Class of Service


Equal Cost Multipath


Ethernet VPN


Forwarding Adjacency


Intermediate System-to-Intermediate System


Label Distribution Protocol


Link State Database


Label Switched Path


Multiprotocol Label Switching


Open Shortest Path First


Penultimate Hop Pop


Resource Reservation Protocol - Traffic Engineering


Traffic Engineering Database


Ultimate Hop Pop


Virtual Private Network


For decades multiprotocol label switched (MPLS) networks have been pushing, popping, and swapping countless labels on untold packets. Labels still remain the fundamental MPLS forwarding instruction. A label-switched path (LSP) involves a contiguous set of routers where, at the simplest level, each performs one or more of the same three forwarding actions —a push, pop, or swap.

An LSP can be handcrafted, with each router’s management plane configured with the appropriate action. More commonly, control plane signaling based on LDP, RSVP-TE, or BGP-LU, weaves an LSP. Neither protocol is mutually exclusive and some of the most demanding networks use a combination.

Segment routing (SR) introduces a fourth option to this trio of well-known label distribution protocols and it does this without resorting to specifying yet another protocol. SR prefers instead to overload existing interior and exterior gateway protocols – IS-IS, OSPF, and BGP. Their protocol machinery is reused with a combination of new attributes that disseminate information about segments, SR’s atomic-like abstraction of a forwarding instruction.

By using segments, SR decouples itself from MPLS in order to be forwarding plane agnostic. SR-over-UDP uses the well-treaded IPv4 forwarding plane and is under active development. SRv6 and SRv6+, whose forwarding plane is IPv6, are being deliberated over by standards bodies. That said, SR with an MPLS forwarding plane remains in its most approachable form. The familiar push, pop, and swap actions are referred to as push, next, and continue, respectively, in SR parlance.

In addition to forwarding plane independence, SR proposes novel and attractive approaches to traffic protection and engineering. Consequently, it delivers both the equal-cost multipathing (ECMP) capabilities of IP routing, along with the ability to precisely engineer a path à la RSVP-TE.

In spite of these desirable qualities, introducing SR into an existing MPLS network of any worth is a sizeable undertaking. Multiple label distribution protocols will be active in the network – at least temporarily – and the timespan is likely to be more in the order of months, not weeks, or days. On some routers, proto- cols will overlap and a configuration will be needed to indicate which is preferred.

The undertaking is well worth the effort. Far from simply reigniting the debate about how networks should route, segment routing ushers in novel approaches to long-standing practices of node identification, network partitioning, and multipathing.


For the reader entirely unfamiliar with SR, the first few pages of Connectivity chapter provide an extremely compressed introduction. For a complete introduction, read the SR companion book, Day One: Configuring Segment Routing with Junos, by Lucek and Szarkowicz, at training/jnbooks/day-one/configuring-segment-routing-junos/