Segment Routing (SRv6)
The Future of Simplified Source Routing
The Paradigm Shift: Source Routing
Imagine you are going on a road trip. Traditional Routing is like driving and stopping at every intersection to ask a local for directions — each router consults its FIB and makes an independent decision based on the destination address alone. Segment Routing is like having a GPS that prints a list of waypoints on your windshield before you leave: "Turn right at Node A, then left at Node B, detour through the firewall at Node C." The source knows the full path; intermediate nodes simply follow the instructions.
This "source routing" principle eliminates the need for routers in the middle of the network to maintain per-flow state, dramatically simplifying the core while providing the source with unprecedented traffic engineering control.
SRv6 Packet Structure
Unlike SR-MPLS, which uses a stack of MPLS labels prepended before the IP header, SRv6 uses the standard IPv6 Routing Header (Type 4) — also called the Segment Routing Header (SRH). This means SRv6 packets are valid IPv6 packets and can cross any IPv6-capable network without modification.
- Segment List: An array of IPv6 SIDs representing the desired path, stored in reverse order (destination first).
- Segments Left: A pointer (decremented at each hop) that indicates which SID in the list is currently active — analogous to the MPLS label at the top of the stack.
SRv6 Network Programming
Watch how SIDs guide a packet using IPv6 Extension Headers.
Operational Insight
Ingress router encapsulates IP packet in SRv6 header. Destination Address (DA) is set to the first Segment ID (SID).
Why SRv6? (Eliminating Complexity)
- No LDP/RSVP: Eliminates two of the most complex protocols in the Service Provider core. LDP distributes label bindings for every IP prefix; RSVP-TE maintains per-tunnel state signaling on every node in the path. SRv6 eliminates both with a single control-plane extension to IS-IS or OSPF.
- TI-LFA (Topology Independent Loop-Free Alternate): Provides guaranteed <50ms protection against link or node failures for any arbitrary network topology — without requiring a pre-provisioned backup LSP. The backup path is computed using post-convergence SPF results.
- Service Chaining: Forces traffic through specific security (firewall), optimization (WAN accelerator), or NAT VNFs simply by including their SIDs in the segment list. No MPLS VPN tunnels or PBR required.
- Network Slicing: Enables distinct "Network Slices" where high-priority traffic (video conferencing, industrial control) takes a low-latency path, while bulk data (backups, analytics) takes a cheaper, high-bandwidth path — all on the same physical infrastructure.
Conclusion
SRv6 is more than a routing protocol; it is a Network Programming Language. It treats the entire global network as a single programmable entity, allowing path steering, service insertion, and traffic engineering to be expressed as a simple list of IPv6 addresses. For operators seeking to simplify their WAN, enable 5G network slicing, or build cloud-scale fabrics, SRv6 is the architecture that makes it operationally feasible.