What is Segment Routing?

Terry Slattery
Principal Architect

Do you remember source routing, used in token ring bridging networks? If you’re relatively new to networking, you may not know about it. In source routing, the hosts determine the path to take through the network and build a frame header that specifies the forwarding nodes that the frame should pass through as it goes from source to destination. The token ring implementation typically used special frames called explorer packets to discover the set of paths from source to destination. Too many explorer packets could cause overloading problems with the network nodes (source route bridges) and with the hosts that had to process them, much like a broadcast storm in Ethernet networks.

Segment Routing

When I first heard about segment routing and that it was based on source routing technology, I was initially curious, because of my history with token ring source route bridging. But fortunately, segment routing is completely different. It doesn’t use explorer packets and it isn’t bridging.

I’ve written a good deal about segment routing, so a good introduction to this subject would be those posts. A great start is my post at nojitter.com, where I write a monthly article.

Then move on to this post at Tech Target. It includes more information about how applications and the network can communicate with each other to select optimum traffic paths. I also recommend a white paper on the technology from Cisco.

Finally, there are more links at the Segment Routing Tech Field Day site.


A number of new innovations to networking are being created and the network world is in a state of transition. It will be interesting to see which technologies make it and which ones wind up in the dustbin of history. Segment routing may make it because of its scalability over other technologies.

Leave a Reply


Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.