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Software-Defined WAN (SD-WAN) is the second “software-defined” marketing attempt (after the original SDN) to dress a conglomerate of old technologies into shiny new clothes. Even Wikipedia article promotes some of the usual software-defined hype, quoting Network World claim that:

SD-WAN simplifies the management and operation of a WAN by decoupling the networking hardware from its control mechanism. This concept is similar to how software-defined networking implements virtualization technology to improve data center management and operation.

Is It Real?

Want to know how real those claims are? Start the journey with this series of myth-busting blog posts:

• Software-Defined WAN:Well-Orchestrated Duct Tape? (2015)
• Routing Protocols and SD-WAN: Apples and Furbies (2015)
• Do Enterprises Need MPLS? (2016)
• Lack of Fast Convergence in SD-WAN Products (2018)
• Lock-In and SD-WAN: a Match Made in Heaven (2019)
• Impact of Controller Failures in Software-Defined Networks (2019)
• Fast Failover in SD-WAN Networks (2020)

Does SD-WAN make sense? Sure:

• Business Case for SD-WAN (2015)
• Reliability of SD-WAN and Hybrid WAN Solutions (2015)

Need More Details?

I covered the basics of SD-WAN in Choose the Optimal VPN Service and SDN Use Cases webinars.

Pradosh Mohapatra described the basics of SD-WAN and its typical components and architectures:

• What Is SD-WAN?
• SD-WAN Reference Design
• SD-WAN Backend Architecture
• SD-WAN CPE Architecture
• Security Aspects of SD-WAN

Want to know more about Cisco’s SD-WAN solution (formerly known as Viptela)? Enjoy David Peñaloza Seijas’ deep dive into its architecture and implementation details:

• Going Beneath the Cisco SD-WAN Surface
• Cisco SD-WAN Fundamentals and Definitions
• Cisco SD-WAN Solution Architecture and Components
• Cisco SD-WAN Routing Goodness
• Cisco SD-WAN Onboarding Process
• Cisco SD-WAN Policies and Centralized Magic
• Cisco SD-WAN Policies Review
• Cisco SD-WAN Routing Design
• Cisco SD-WAN Site Design
• Cisco SD-WAN Policy Design
• Automating netlab-Based Cisco SD-WAN Deployment

Real-Life SD-WAN

SD-WAN sounds great, but does it work as expected? Maybe not:

• SDN, SD-WAN and FCoE on Gartner Networking Hype Cycle (2015)
• SD-WAN Reality Gap (2019)
• Real-Life SD-WAN Experience (2019)
• Worth Reading: SD-WAN Scalability Challenges (2020)
• Feedback from Another SD-WAN Fan (2020)

Is it secure? Some products seem to be nothing more than a bunch of open-source component glued together with clueless Python code:

• Security Aspects of SD-WAN Solutions (2018)
• SD-WAN Security Under the Hood (2019)
• SD-WAN Security: A Product Liability Insurance Law Would Certainly Help (2020)
• Another SD-WAN Security SNAFU: SQL Injections in Cisco SD-WAN Admin Interface (2021)

Some service providers want to use SD-WAN to offer managed services. Not surprisingly, some people¹ don’t find that a good idea:

• SD-WAN: A Service Provider Perspective (2020)
• Managed SD-WAN Services (2022)
• Challenges of Managed SD-WAN Services (2022)

Then there are some technical details vendors love to gloss over:

• Does Unequal-Cost Multipathing Make Sense? (2021)
• Topology- and Congestion-Driven Load Balancing (2021)

Does it work within a public cloud? Yeah, sort of… with a few challenges:

• Azure Route Server: The Challenge (2021)
• Azure Route Server: Behind the Scenes (2021)

Want Even More?

Love marketing-related rants? Here are a few:

• Some Ridiculous SD-WAN Claims (2015)
• What Is Software-Defined Security? (2016)
• This Is Why I’m Not Doing SD-WAN Webinars (2016)
• The Ever-Increasing Complexity (2017)
• SD-WAN Vendor Landscape (2019)

Last, but definitely not least, you might enjoy these (more esoteric) solutions:

• DLSP – QoS-Aware Routing Protocol on Software Gone Wild (2015)
• Changing Cisco IOS BGP Policies Based on IP SLA Measurements (2019)
• Overlay Networking with Ouroboros on Software Gone Wild (2020)
• Scalable Policy Routing (2021)

Blog Posts I Forgot to Categorize

• Does EVPN/VXLAN over SD-WAN Make Sense?

I firmly believe that you cannot be a good networking engineer¹ without a firm grasp of the networking fundamentals, and I couldn’t resist pointing that out a few times (see also certifications-related posts):

Regardless of how far down this page you’ll get, these blog posts are a must-read:

I would also suggest exploring these series of blog posts as well as textbooks and other resources I collected:

• Interfaces and Ports
• Packet Forwarding Basics
• Integrated Routing and Bridging (IRB) Designs
• IP Anycast and Anycast Gateways
• Site and Host Multihoming
• High Availability Switching
• Fast Failover
• Unnumbered IPv4 Interfaces
• CLI versus API
• Network State Consistency

The rest of the fundamentals-related blog posts are collected on this page.

Contents

Network Addressing

Addresses and routes are the basic concepts anyone dealing with a network must (eventually) grasp. These blog posts describe how we got a hierarchy of addresses:

I also compared the device addresses (used in OSI) and interface addresses (used in TCP/IP):

Bridging, Routing, and Switching

There is a single reason we build computer networks (apart from job security): we want them to transport data between the attached endpoints. These blog posts describe some of the interesting details:

Deep Dives

These blog posts dive deeper into interesting topics:

If you like them, it’s probably time you start exploring the deep-dive series I already mentioned.

A Bit of a History

These blog posts might help you figure out some less obvious details or give you a historical perspective on why networking technologies evolved to where we are right now:

If you want to dive deeper into historical technologies, you might enjoy the comparison of TCP/IP and OSI (CLNP) protocol stacks:

There Be Rants

Long-time readers know I can’t resist a good rant:

Everything Is a Graph

You can represent every network as a graph of network devices (nodes) and links². Rachel Traylor covered the graph theory in the (free) Network Connectivity, Graph Theory, and Reliable Network Design and Graph Algorithms in Networks webinars; these blog posts might provide some extra details:

Networking Fundamentals Videos

Finally, I published dozens of videos describing the networking concepts as part of the How Networks Really Work webinar that got at least some minor positive feedback. The videos describe:

Business aspects of networking technologies

Some people liked the non-technical take on networking I recorded in 2019 and 2020:

Fallacies of distributed computing

Networking challenges and the importance of a layered approach

Network Addressing

Switching, Routing, and Bridging

Routing Protocols

Lessons Learned from 35 Years of Networking

Other Blog Posts in This Category

2026-01

• On MPLS Paths, Tunnels and Interfaces

2025-06

• Finding Source Routing Paths
• Finding End-to-End Paths: Topology and Endpoints

2025-05

• Multi-Layer Switching and Tunneling
• Forwarding Packets Across a Network

2025-04

• Terminology
• Local (Node) Multihoming

2025-03

• Network State Summarization
• Reaching Off-Subnet Nodes

2024-10

• Finding Adjacent Nodes

2024-03

• OSI Layers in Routing Protocols

2024-02

• Network Layer: Interface or Node Addresses
• Can We Skip the Network Layer?

2024-01

• On Routing Protocol Metrics

2023-11

• LAN Data Link Layer Addressing
• Fibre Channel Addressing

2023-10

• Early Data-Link Layer Addressing

2023-09

• Why Do We Need Source IP Addresses in IP Headers?
• Addresses in a Networking Stack
• Names, Addresses, and Routes

2023-06

• Worth Reading: A Primer on Communication Fundamentals
• Link State Routing Protocol Implementations

2023-02

• Link State Routing Protocol Basics

2023-01

• Relationships between Layer-2 (VLAN) and Layer-3 (Subnet) Segments

2022-11

• Routing Protocols Overview

2022-09

• The Basics of Network Address Translation (NAT)
• Was IPv6 Really the Worst Decision Ever?

2022-06

• When You Find Yourself on Mount Stupid
• Network Address Scopes

2022-05

• Network Address Assignments

2022-04

• Combining Data-Link- and Network Layer Addresses

2022-03

• Comparing TCP/IP and CLNP

2022-02

• Network Layer Addressing
• Lesson Learned: The Way Forward

2022-01

• Local Area Network Addressing

2021-11

• Some Services Are Not Worth Delivering
• Early Data-Link-Layer Addressing
• Business Aspects of Networking

2021-10

• Feedback: How Networks Really Work
• Lessons Learned: Complexity Will Kill Your System
• Theoretical View of Network Addressing

2021-09

• Lessons Learned: Fundamentals Haven't Changed
• Introduction to Network Addressing

2021-06

• Typical Large-Scale Bridging Use Cases
• Comparing Routing and Bridging
• Lessons Learned: Technology Still Matters
• Real-Life Network-as-a-Graph Examples
• Routing Protocols: Use the Best Tool for the Job

2021-05

• IP Routing Fundamentals

2021-04

• Everything Is a Graph
• Response: Is Switching Latency Relevant?
• Transparent Bridging Fundamentals
• Is Switching Latency Relevant?

2021-03

• Path Discovery in Transparent Bridging and Routing

2021-02

• High-Level Technology Guidelines
• OMG, It's Graphs Everywhere
• Finding Paths Across the Network

2021-01

• Multi-Layer Switching and Tunneling
• IBGP, IGP Metrics, and Administrative Distances

2020-12

• Learning Networking Fundamentals at University?
• Should You Build or Buy a Solution?
• Chasing CRC Errors in a Data Center Fabric

2020-11

• Know Your Users' Needs
• Why Is OSPF not Using TCP?
• Getting a Packet Across a Network
• Appreciating the Networking Fundamentals

2020-10

• Grasp the Fundamentals before Spreading Opinions

2020-09

• What Are Bridging, Routing, and Switching?
• Define the Problem Before Searching for a Solution

2020-08

• Networks Are Not Homogenous
• Review Questions: Switching, Bridging and Routing
• IPv4, IPv6, and a Sudden Change in Attitude

2020-05

• Feedback: How Networks Really Work
• Internet Has More than One Administrator

2020-04

• Networks Are (Not) Secure

2020-03

• The World in Which IPv6 Was a Good Design
• Bandwidth Is Neither Infinite Nor Cheap

2020-02

• End-to-End Latency Is Not Zero
• The Network Is Not Reliable

2020-01

• Fallacies of Distributed Computing

2019-11

• Breaking the End-to-End Principle
• Putting the Networking Layers Together

2019-10

• How Did We End With 1500-Byte MTU?
• Retransmissions and Flow Control in Computer Networks

2019-09

• On the Usability of OSI Layered Networking Model
• The Need for Network Layers
• Beyond Two Nodes
• Response: The OSI Model Is a Lie

2019-08

• Introducing Transmission Technologies
• Overview of Networking Challenges

2015-03

• You Must Understand the Fundamentals to Be Successful

2011-02

• How Did We Ever Get Into This Switching Mess?

2010-07

• Bridges: a Kludge that Shouldn't Exist

2009-12

• Lies, damned lies and product marketing

2009-08

• The Awful Socket API

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1. In the stricter sense, not in the “every CLI jockey is called an engineer these days” one ↩︎

2. Multi-access networks are represented as pseudo-nodes ↩︎