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Segment routing

Resilient, scalable and simplified packet transport.

What is segment routing?

Segment routing is a packet steering technology that offers a highly scalable approach for establishing predefined forwarding paths in the IP network while simplifying the control plane. Segment routing offers benefits to shortest path forwarding by improving fault coverage and enhances the traffic engineering of paths to meet specific constraints such as available bandwidth, latency, protection or physical diversity.  

Segment routing steers packets by encoding them, at ingress, with Segment Identifiers (SIDs) that contain the packet-processing instructions for each intermediate and destination router. This greatly reduces the need for a control plane to instantiate and maintain path state in the network, which simplifies network operations and reduces network resource requirements. 

What are the benefits of segment routing?

Make traffic engineering more scalable

Segment routing offers a highly scalable approach for establishing predefined, deterministic forwarding paths in the IP network. As the paths are defined at ingress, there is a significant reduction in soft state held in the network and in signaling between network elements. This reduces network resource requirements such as memory, bandwidth and processing capacity.

Improving reliability and resilience

Segment routing supports local and end-to-end path protection and restoration. It can be applied to pre-calculate backup paths that rapidly protect against the failure of the primary path. In ring and mesh topologies, segment routing improves reliability, resilience and service availability.

Automate and simplify operations

Segment routing was designed with automation in mind and supports auto-configuration, intent-based networking and model-driven management (MDM) operations. It simplifies network operations by avoiding the need for additional signaling protocols (e.g., LDP, RSVP-TE). Segment routing also provides a clear and seamless evolutionary path from LDP/RSVP-TE.

Create new revenue streams

In supporting high-scale, efficient, resilient and automated traffic-engineered paths, segment routing can form an infrastructure for a wealth of network services that could unlock new revenue. For example, segment routing can support seamless end-to-end transport network slicing from the data center to the access network.

Why Nokia for segment routing?

Nokia supports a comprehensive segment routing capability, including both segment routing with MPLS  (SR-MPLS), and with IPv6 (SRv6), forwarding planes. SR-MPLS combines all the proven attributes of MPLS protocols into a single framework, it represents a field-hardened choice for many IPv4 and IPv6 transport networks.  SRv6 is gaining strong traction, particularly in edge data center and new IPv6 backbone use cases, it provides a powerful framework for programmability of IPv6 networks that takes advantage of the large IPv6 address space. Compressed micro SIDs are used to reduce large SRv6 header sizes and associated resource usage. 

Nokia’s implementation of SR-MPLS and SRv6 builds on our extensive global deployment experience in high scale IP networks. The application note: Segment routing on Nokia routing platforms describes how Nokia efficiently supports scalable and resilient services with highly automated operations on a segment routing infrastructure  with an SR-MPLS or SRv6 forwarding plane.   Seamless network evolution is supported by our innovative interworking gateway capabilities, allowing your network evolution to be driven by business goals, unconstrained by technology. 

Paths for segment routes can be conveyed to a headend router in various ways, such as model driven CLI, NETCONF/YANG, Border Gateway Protocol (BGP) updates or from a Path Computation Element (PCE). A PCE provides automated global traffic optimization for highly efficient network operation combined with optimal Service Level Agreement (SLA) adherence. The application note: Path control and optimization in segment routed networks describes how Nokia’s Network Services Platform  (NSP) acts as a PCE to delivers automated path control and optimization. As a comprehensive automation platform, NSP also delivers a suite of ready-to-use applications that help network operations teams deploy a rich set of use cases for network management, orchestration and control.

What are the principal capabilities of segment routing ?

Operators can incrementally add the segment routing features they need to an existing LDP/RSVP network with the options they’re comfortable with. This eases the migration to segment routing and enables them to gain operational experience before introducing more powerful features.


The graphic above shows an incremental approach to deploying the principal applications of segment routing.

Shortest path routing is a good starting point for introducing segment routing as a replacement for LDP to enable better protection coverage. It only requires control plane extensions for IS-IS or OSPF. Global segment routing label blocks and node and link adjacencies can be configured for each router, after which SR-IS-IS or SR-OSPF tunnels can be used by IP services.  

Constraint-based forwarding could be a logical next step because it enables more granular programming of the network, for example, to engineer low-latency paths for delay-sensitive applications, or to ensure that traffic flows are kept within a controlled set of links (data sovereignty). 

By augmenting the segment routing shortest path with an IGP flexible algorithm (Flex-Algo), it becomes possible to add topology constraints and alternative link metrics to the shortest path calculation. This application invokes segment routing traffic engineering capabilities (SR Policy and Flex-Algo) and programmatic control, for example, with BGP and BGP-Link State (BGP-LS).

SR-TE capabilities can be further extended with BGP and BGP-LS to include BGP prefix and peer segments, and steer traffic to a particular egress point (i.e., egress peer engineering).

Path diversity and end-to-end protection enable highly available premium services as a more scalable alternative for MPLS fast reroute based on RSVP-TE. SR-TE LSPs with diverse primary and secondary paths are enabled by including Shared Risk Link Group (SRLG) constraints for the candidate paths.

Seamless Bidirectional Forwarding Detection (BFD) is used to quickly detect failures, including silent failures that are not visible to the control plane, and to trigger a failover to the secondary path if the primary path fails. 

Peering engineering enables a headend router to steer traffic across a specific downstream peering link between two IGP domains. This is useful for applications such as traffic optimization, resiliency or load balancing across links that interconnect domains.

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