What is SRv6 (Segment Routing IPv6)? Part 1


In today's rapidly evolving networking landscape, the need for efficient and flexible network routing solutions has never been more critical. Enter SRv6, a groundbreaking technology that combines the power of Segment Routing (SR) with IPv6 to revolutionize the way networks are built and managed. In this comprehensive technical blog, we will delve deep into the world of SRv6, exploring its architecture, key features, benefits, and use cases. By the end of this journey, you will have a clear understanding of how SRv6 can transform your network infrastructure.

What is SRv6?

Segment Routing IPv6 (SRv6) is an innovative network architecture that leverages the existing IPv6 infrastructure to enable efficient packet forwarding and traffic engineering. It introduces a new routing paradigm, where the network forwarding path is determined by a Segment Identifier (SID) stack embedded in the IPv6 packet header.

SRH (Segment Routing Header) in Network Routing

SRv6 Header:

The SRv6 (Segment Routing with IPv6) header is an extension to the IPv6 header. It consists of multiple segments, which are represented by Segment Identifiers (SIDs). These SIDs define the forwarding instructions for packets as they traverse the network.

The SRv6 header is inserted between the IPv6 header and the upper-layer protocol header (e.g., TCP, UDP). It carries crucial information that guides routers in forwarding packets along specific paths defined by segments.

The Segment Routing Header (SRH):

SRv6 Packet Structure:

An SRv6 packet consists of the following components:

Structure and Fields of SRH:

1. Next Header: - The Next Header field in SRH indicates the type of the next header in the packet, typically representing the upper-layer protocol.

2. Hdr Ext Len: - Hdr Ext Len denotes the length of the SRH header, including any optional segments and padding.

3. Routing Type: - The Routing Type field specifies the type of routing used in SRH. In the case of SRv6, it is set to a specific value to indicate the use of SRH.

4. Segments Left: - Segments Left field represents the number of remaining segments in the segment list. It gets decremented as the packet traverses routers.

5. Last Entry: - The Last Entry flag indicates the final entry in the segment list, helping routers identify the end of the segment routing path.

6. Flags: - Flags field provides additional control and options related to SRH.

7. Tag: - The optional Tag field carries supplementary information or context relevant to the segment routing path.

8. Segment List: - The Segment List is a sequence of Segment Identifiers (SIDs) that define the ordered path for packet forwarding. Each SID represents a specific operation or behavior to be executed at the corresponding router.

Segment Identifiers (SIDs)

SRv6 makes use of Segment Identifiers (SIDs) to define the forwarding behavior of packets as they traverse the network. SIDs are essentially IPv6 addresses that are used to represent specific segments or instructions in the network. There are several types of SIDs used in SRv6, each serving a different purpose. Here are some commonly used types of SIDs:

1.End SID (End.X): The End SID represents the endpoint of a segment. When a packet reaches an End SID, it is delivered to the node identified by the SID. This can be used to steer traffic to specific network devices or services.

2.End.X with a specific IPv6 address: This type of SID is similar to the End SID but specifies a specific IPv6 address as the endpoint of the segment.

3.End.T (Traffic Engineering SID): The End.T SID allows for traffic engineering capabilities. It can be used to apply traffic engineering policies, such as steering traffic through specific paths or enforcing specific quality-of-service (QoS) requirements.

4.End.DX2 (Decapsulation and specific IPv6 address): The End.DX2 SID is used for decapsulation operations, where the outer IPv6 header is removed, and the packet is delivered to the specific IPv6 address indicated in the SID.

5.End.DX4 (Decapsulation and specific IPv4 address): Similar to End.DX2, the End.DX4 SID is used for decapsulation operations, but the packet is delivered to a specific IPv4 address instead.

6.End.DT4 (Decapsulation, translation, and specific IPv4 address): The End.DT4 SID performs decapsulation, translation from IPv6 to IPv4, and delivers the packet to a specific IPv4 address.

These are just a few examples of the types of SIDs used in SRv6. The specific set of SIDs used in a network depends on the network design, requirements, and the SRv6 implementation being used.

SRH (Segment Routing Header) TLVs mentioned

1. End.T (Endpoint): - This TLV indicates the endpoint of the segment routing path. It helps routers identify the final destination for the packet. When a router encounters this TLV, it knows that it has reached the endpoint and can deliver the packet accordingly.

2. End.X (Endpoint with Exclusion): - The End.X TLV allows for excluding specific segments from the segment routing path. It provides granular control over the segments to be bypassed. Routers receiving this TLV will skip the excluded segments and continue forwarding the packet to the next specified segment.

3. End.S (Endpoint with Source Routing): - This TLV instructs routers to perform source routing based on the specified segments. It enables explicit routing and precise control over the packet's path. Routers follow the specified segments in order to forward the packet along the designated path.

4. End.DT (Endpoint with Decryption and Traffic Engineered Path): - The End.DT TLV combines decryption and traffic engineering capabilities. It specifies that the packet should be decrypted at a particular segment and then forwarded along a traffic-engineered path. This TLV is useful when specific decryption or encryption operations are required at certain points in the path.

5. End.DX (Endpoint with Decryption and Exclusion): - This TLV combines decryption and exclusion functionalities. It instructs routers to exclude specific segments and decrypt the packet at the designated endpoint. It allows for selective exclusion of segments and subsequent decryption, providing flexibility in packet processing.

6. End.B6 (Endpoint with Binding SID): - The End.B6 TLV associates a Binding SID with the endpoint of the segment routing path. It helps routers determine the appropriate forwarding action based on the Binding SID. When a router encounters this TLV, it looks up the Binding SID to determine the next hop or forwarding behavior.

7. End.V (Endpoint with Virtualization): - This TLV is used in virtualized network environments. It provides information related to virtual network functions (VNFs) or virtual routers associated with the endpoint. It allows for seamless integration and forwarding of packets within virtualized network infrastructures.

8. Pad TLV (Padding): - The Pad TLV is used to add additional padding to the SRH. It ensures proper alignment of the header on a specific boundary, which can be important for efficient processing by routers.

9. Binding TLV: - The Binding TLV provides information about the binding between a segment and a specific resource or service. It helps routers determine the appropriate forwarding behavior based on the binding information provided.

10. SID List TLV (Segment Identifier List): - The SID List TLV enables the inclusion of multiple segments in a single TLV. It allows for specifying a list of segments that need to be traversed in the segment routing path. Routers process the segments in the order specified in the list.

11. MPLS Label Stack TLV: - The MPLS Label Stack TLV carries the MPLS label stack to be imposed on the packet at each segment along the path. It provides the necessary information for label-based forwarding in MPLS-based segment routing deployments.

12. Time To Live (TTL) TLV: - The TTL TLV specifies the time to live value for the segment routing path. It helps control the lifespan of the packet and prevent it from circulating indefinitely in the network. Routers decrement the TTL value as the packet progresses through the path.

13. Traffic Class TLV: - The Traffic Class TLV allows for specifying the desired traffic class or quality of service (QoS) treatment for the packet along the segment routing path. It helps prioritize or differentiate traffic based on specific requirements.

14. Fast Reroute TLV: - The Fast Reroute TLV provides instructions for fast rerouting mechanisms in case of link or node failures. It helps ensure high availability and resilience in segment routing networks by specifying alternate paths or actions to be taken in the event of failures.

These TLVs enhance the capabilities of SRH by providing additional instructions, control, and flexibility in segment routing. They allow for more granular control over packet forwarding, support advanced functionalities such as encryption/decryption, traffic engineering, virtualization, and enable fast rerouting for improved network resilience.

Benefits of SRH:

1. Flexibility and Programmability: - SRH enables dynamic path selection and traffic engineering, allowing network operators to define and adapt routes based on changing requirements.

2. Efficient Resource Utilization: - By leveraging SRH, routers can make intelligent forwarding decisions based on segment instructions, optimizing resource utilization and reducing network congestion.

3. Service Chaining and Virtual Network Function (VNF) Steering: - SRH empowers service chaining, enabling the seamless insertion of network services or Virtual Network Functions (VNFs) along the segment routing path.

4. Scalability and Simplified Operations: - With SRH, networks can scale efficiently while reducing the complexity of traditional routing protocols, enhancing network manageability and operational efficiency.

Real-World Use Cases::

1. Traffic Engineering and Optimization: - SRH allows for fine-grained control over traffic paths, enabling network operators to optimize network performance based on metrics such as latency, bandwidth, and link utilization.

2. Multi-Cloud Connectivity: - By leveraging SRH, organizations can establish robust and efficient connectivity between multiple cloud environments, facilitating seamless workload migration and hybrid cloud architectures.

3. Service Function Chaining: - SRH enables service chaining, allowing the insertion of services such as firewalls, load balancers, and deep packet inspection at specific points along the segment routing path.


In conclusion, the introduction of the Segment Routing Header (SRH) has ushered in a new era of network routing capabilities. With SRH, networks gain unprecedented control, flexibility, and efficiency, transforming the way organizations design and operate their networks. By leveraging SRH's ability to define explicit paths and enable dynamic traffic engineering, businesses can optimize their network performance, simplify operational complexities, and deliver enhanced user experiences.

This blog has served as Part 1 of our exploration into the world of SRv6 (Segment Routing IPv6). We have delved into the fundamentals of SRH and its potential to revolutionize network routing. Understanding the details of SRv6 is key to harnessing its power for network optimization and efficient traffic management.

We hope you found this introduction to SRv6 insightful and informative. Stay tuned for Part 2, where we will dive deeper into practical implementations, real-world use cases, and the benefits of adopting SRv6 in your network infrastructure. Happy reading and stay ahead of the curve in network routing innovation!