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...

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...

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...

The book is designed in such a manner that it takes you from a traditional MPLS network to an SR-MPLS network by introducing relevant features with theory as well as practical labs:

• Lab 1, Getting Started with LDP-Based MPLS Network, details the initial steps for you, explaining the IP schema, configuring IGP, and setting up traditional MPLS. This lab serves as the foundation for the subsequent chapters.
• Lab 2, Introducing Segment Routing MPLS (SR-MPLS), breaks down the components of SR from both control and data plane perspectives, providing a comprehensive understanding of its building blocks.
• Lab 3, SR-LDP Interworking, explores the coexistence of SR and LDP as separate domains within the same IGP backbone. It covers forwarding traffic between SR and LDP domains.
• Lab 4, Introducing TI-LFA (Topology Independent - Loop-Free Alternate), introduces the fast-reroute backup path mechanism, offering a means to protect SR-MPLS traffic in the core...

At its core, a network infrastructure comprises a combination of physical and virtual components that possess the ability to establish connectivity among themselves, facilitating the seamless flow of traffic. These elements include routers, switches, servers, and more. The size and complexity of the network infrastructure depend on the network’s requirements and the scale of its operations.

In the lab described in this book, building blocks refer to the important parts or tools needed to set up the network system for education or learning. These building blocks are like the basic building materials that form the base of the lab, including specific software and hardware components.

The lab’s building blocks, as described in the book, include the following:

• Network operating system: For the creation of network topologies in this book, the chosen network operating system is Cisco IOS XR, specifically using the XRv9k virtual image, a commercial...

By now, you will have acquired a comprehensive grasp of MPLS and its pivotal role in modern networking. Fundamental MPLS concepts, spanning its definition, applications, and key features such as FRR and SR, have been thoroughly explored.

MPLS emerges as a versatile tool, catering to diverse networking needs, including efficient traffic management, enhanced QoS, and robust VPN support.

SR represents a paradigm shift in routing architectures, offering streamlined network operations and improved scalability while maintaining existing data-plane forwarding mechanisms. SR-MPLS seamlessly integrates SR with traditional MPLS networks, facilitating eventual migration to SR-only MPLS environments without major disruption.

FRR stands out as a critical element in enhancing network resilience, proactively establishing alternative paths that can swiftly activate in response to network failures.

The chapter has also outlined the network infrastructure tailored for the book’...

The following are the references used in this chapter, offering additional resources and further reading to deepen your understanding of the concepts discussed:

• https://www.segment-routing.net/
• https://www.cisco.com
• https://containerlab.dev/
• https://www.eve-ng.net/
• https://cloud.google.com/
• Segment Routing Part I, by Clarence Filsfils, Kris Michielsen, Ketan Talaulikar
• [RFC1180] Socolofsky, T. and C. Kale, “TCP/IP tutorial”, RFC 1180, January 1991, <http://www.rfc-editor.org/info/rfc1180>.