IPv6 Source Routing · SRH Extension Header

Segment Routing over IPv6

SR applied to the IPv6 data plane: segment lists encoded as a sequence of IPv6 addresses in an SRH extension header, enabling source routing without MPLS labels.

SRv6 SID List Forwarding

A packet traverses three SR endpoints.

SRv6 packet flow through three endpoints with IPv6 SID list fc00::1, fc00::2, fc00::3
SRv6: packet traversal through three endpoints driven by an IPv6 SID list.

What is Segment Routing?

Segment Routing (RFC 8402) is a source-routing architecture. The head-end encodes the path a packet takes as an ordered list of instructions called segments, and the network forwards on that list without holding per-flow state at every hop. It runs on two data planes: SR-MPLS carries the segment list as an MPLS label stack, and SRv6 carries it as a list of IPv6 addresses. This page covers SRv6, and the SR-MPLS data plane is documented on its own page.

What SRv6 Is

SRv6 (RFC 8986) encodes a source-initiated path as an ordered list of IPv6 addresses called SIDs, each representing a topological instruction (segment) at a specific node. The Segment Routing Header (SRH, RFC 8754) carries the SID list in the IPv6 extension header stack. At each SR node, the active SID is processed, the SL (Segments Left) pointer is decremented, and the IPv6 DA is updated to the next SID.

SRv6 SIDs are routable IPv6 addresses structured with a Locator (topologically significant prefix, typically /48 or /64) and a Function (behavior identifier). This means SR forwarding state is embedded in the IPv6 routing table: no separate label space, no LFIB, no MPLS-specific HW resources. Transit nodes need no SRH awareness unless they are SR endpoints.

SRv6 uSID (draft-ietf-spring-srv6-srh-compression) compresses the SID encoding: several micro-SIDs are packed into a single 128-bit IPv6 address, which shortens the SID list and reduces header overhead on long engineered paths.

SRv6 Network Programming

RFC 8986 defines a set of SRv6 endpoint behaviors: End (SR endpoint), End.X (endpoint with cross-connect), End.T (endpoint with table lookup), End.DT4/DT6/DT46 (endpoint with decap and L3 table lookup). These behaviors implement the full VPN forwarding function within the SID semantics, enabling L3VPN and L2VPN services over a pure IPv6 core.

Segment Routing in OcNOS-SP

OcNOS-SP runs SR-MPLS on all 26 of its routers. SRv6 support is initial, on selected routers, with more in development.

SR-MPLS on All 26 Routers

IS-IS and OSPF segment routing extensions, TI-LFA fast reroute, IS-IS Flex-Algo and SR Policy on every OcNOS-SP router.

IS-IS and OSPF for SRv6

IS-IS and OSPF extensions for segment routing over the IPv6 data plane, on ten Qumran 2 family routers with the PLUS edition.

Initial SRv6 Services

BGP L3VPN (VPNv4), EVPN E-LINE and single-homed EVPN E-LAN over SRv6, signaled per RFC 9252.

SRv6 OAM

SRv6 OAM per RFC 9259 on the same routers.

Routers with Initial SRv6 Support

For reference only. Initial SRv6 support runs on ten Qumran 2 family OcNOS-SP routers with the PLUS edition, and seven of them are shown below. The OcNOS Feature Matrix lists current support per router and release.

UfiSpace S9600-56DX
Qumran2c · 4×25G + 48×100G + 8×400G
UfiSpace S9600-28DX
Qumran2c · 4×25G + 24×100G + 4×400G
UfiSpace S9600-32X
Qumran2c · 32×100G
UfiSpace S9600-64X
Qumran2c · 64×100G
UfiSpace S9510-28DC
Qumran2a · 24×25G + 2×100G + 2×400G
Edgecore AS7535-28XB
Qumran2a · 24×25G + 2×100G + 2×400G
UfiSpace S9510-30XC
Qumran2u · 28×25G + 2×100G
See all platforms in Qualified Hardware → SRv6 in the Feature Matrix →

Related Technologies & Solutions

SR-MPLS → EVPN-VXLAN → BGP → Open IP Core SR-MPLS design → SP Networks Solution → IP-over-DWDM Solution → OcNOS Feature Matrix → Segment Routing explained → MPLS Traffic Engineering (RSVP-TE) → IS-IS SR with TI-LFA in OcNOS →

Frequently asked questions

What is SRv6?
SRv6 (Segment Routing over IPv6) is segment routing that uses the IPv6 data plane directly. Each segment is encoded as a 128-bit IPv6 address called a SID, so the network steers traffic along an engineered path using native IPv6 forwarding, with no MPLS label stack.
What is the difference between SRv6 and SR-MPLS?
SRv6 encodes segments as 128-bit IPv6 addresses and forwards on the IPv6 data plane, while SR-MPLS encodes segments as MPLS labels and forwards on the MPLS data plane. SRv6 needs only IPv6 in the core, where SR-MPLS keeps an MPLS forwarding plane.
What is a uSID in SRv6?
A uSID (micro-SID) packs several SRv6 instructions into a single 128-bit IPv6 address, so one address can carry many segments. This shortens the segment list, reduces packet overhead, and lets longer engineered paths fit within standard IPv6 forwarding.
Does SRv6 need MPLS?
No, SRv6 does not need MPLS. It runs on the native IPv6 data plane and uses IPv6 addresses as segment identifiers, so the core forwards on IPv6 alone. This lets operators run traffic engineering and service routing without an MPLS label-switching plane.
What is the SRH in SRv6?
The SRH (Segment Routing Header) is an IPv6 extension header that carries the ordered list of SIDs a packet must follow. As the packet traverses the path, the active segment in the SRH determines the next hop, encoding the engineered route inside the IPv6 packet itself.
Should I choose SRv6 or SR-MPLS?
SRv6 removes MPLS and runs traffic engineering and VPN services on native IPv6, which suits IPv6-first and greenfield networks. SR-MPLS keeps the MPLS forwarding plane, which suits operators with an installed MPLS base who want segment routing without changing the data plane.
Does SRv6 support L3VPN?
Yes. SRv6 carries L3VPN using the End.DT4, End.DT6, and End.DT46 endpoint behaviors from RFC 8986, which decapsulate a packet and look it up in the target VRF. BGP advertises the per-VPN SIDs as SRv6 services per RFC 9252, so a VPN runs over a pure IPv6 core with no MPLS service label.
Does OcNOS support SRv6?
OcNOS-SP has initial SRv6 support on selected routers, with more in development. It covers BGP L3VPN and EVPN E-LINE and single-homed E-LAN services over SRv6, SRv6 OAM, and the IS-IS and OSPF extensions for SRv6, on ten Qumran 2 family routers with the PLUS edition. For segment routing across all 26 OcNOS-SP routers, OcNOS runs SR-MPLS with TI-LFA fast reroute and IS-IS Flex-Algo.

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