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Overview of Adjacency Segments, Anycast Segments, and Configurable SRGB in SPRING

Segment routing (SR) or Source Packet Routing in Networking (SPRING) is a control-plane architecture that enables an ingress router to steer a packet through a specific set of nodes and links without relying on the intermediate nodes in the network to determine the actual path it should take. SPRING enables automation of a network by using a software-defined network (SDN) controller for traffic steering and traffic engineering in a WAN packet network. To steer packets through the specified set of nodes and links, the ingress router prepends packets with segments that contain an appropriate combination of tunnels. Each segment is associated with an identifier, which is referred to as the segment identifier (SID). An ordered list of segments is encoded as a stack of labels. Every node in the segment routing domain is allocated labels based on the availability of the dynamic label range. A segment routing global block (SRGB) is the range of label values reserved for segment routing.

Configure the SRGB range label for SPRING. These labels are used by SPRING within the IS-IS or OSPF domain. This way the labels advertised in segment routing are more predictable and deterministic across the segment routing domain. To configure the starting index value of the SRGB label block, configure:

  • The start-label start-label-block-value statement at the [edit protocols isis source-packet-routing srgb] hierarchy level for IS-IS.

  • The start-label start-label-block-value statement at the [edit protocols ospf source-packet-routing srgb] hierarchy level for OSPF.

To configure the index range of the SRGB label block, configure:

  • The index-range value statement at the [edit protocols isis source-packet-routing srgb] hierarchy level for IS-IS.

  • The index-range value statement at the [edit protocols isis source-packet-routing srgb] hierarchy level for IS-IS.

Define the SRGB for the IS-IS protocol, and provide prefix anycast segments in addition to node segments to prefixes that are advertised by the IS-IS protocol through policy configuration. Junos OS also extends support to SPRING anycast segments and configurable adjacency segment indexes for the IS-IS protocol.

Benefits of Anycast Segments, Adjacency Segments, and Configurable SRGB

  • With the support for anycast prefix segments on Junos OS, you can configure multiple routers to advertise the same prefix with the same SID, which facilitates load balancing.

  • Configuring the adjacency hold time helps retain segments for a specified period of time after a link flaps and ensures faster convergence after a link fails.

  • Configuring the SRGB label range ensures that the labels are more predictable across segment routing domain.

Configurable Segment Routing Global Block

A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. Every node in the segment routing domain is allocated labels by the node label manager based on the index range configured for source packet routing. These labels are allocated to the node segment based on the availability of the dynamic label range managed by node label manager. An SRGB is the range of label values used in segment routing.

Junos OS supports SRGB under mpls label-range. You can configure an available SRGB label range for the IS-IS, OSPF, and BGP protocols so that the labels are predictable across segment routing domains. Ensure that the configured SRGB labels are not used by any other application.

To configure SRGB under the global MPLS label range, execute the following command: set protocols mpls-label-range srgb range-start end range-end. You don’t need separate label ranges for each protocol because Junos OS centralizes SRGB under MPLS. This enables IS-IS, OSPF, and BGP share one predictable pool, conserving label space

Adjacency Segments and Prefix Segments

A node steers a packet to its destination through an ordered list of instructions, called segments. Essentially, segment routing engages interior gateway protocols (IGPs) such as IS-IS and OSPF to advertise two types of network segments:

  • Adjacency segments—A strict forwarded single-hop tunnel that carries packets over a specific link between two nodes, irrespective of the link cost.

  • Prefix segments—A multihop tunnel that uses equal cost multi-hop aware shortest path links to reach a prefix. The prefix SID supports both IPv4 and IPv6 prefixes. A node segment is a special case of prefix segment, where every node computes shortest path to the node segment and programs in the forwarding plane. An anycast segment is also a type of prefix segment that identifies a set of routers to advertise the same prefix with the same SID value.

You can redistribute Segment Routing (SR) prefix-Segment Identifiers (SIDs) across OSPF IGP instances using route policy without explicitly specifying a prefix-segment index. This feature standardizes SR labels across instances and improves operational efficiency. Configure a policy with the from prefix-segment statement to match routes carrying prefix-segment information. In the then clause, use prefix-segment redistribute to inherit segment information from the matched route. We also support stitching mpls.0 routes to enable interoperability between different IGP instances.

Configurable Adjacency Segment Hold Time

The IS-IS protocol creates adjacency segments per adjacency, level, and address family (one each for IPv4 and IPv6). An MPLS label is allocated for each adjacency segment that gets created. These labels are allocated after the adjacency status of the segment changes to the up state. Configure a hold time to ensure that IS-IS does not release the segments immediately after a link flaps or goes down, but retains them for the configured hold time duration. The default hold time for adjacency segments in IS-IS protocol is 300 seconds.

The OSPF protocol creates adjacency segments per adjacency. To ensure adjacency segments are retained during adjacency or link flaps, the adjacency segments are not released immediately during the link down. The default hold time for adjacency segments in OSPF protocol is 180 seconds.

Prefix Segment Index

Currently, Junos OS enables you to configure a SPRING node SID for IPv4 and IPv6 address families for each routing instance. This node SID is attached to an IPv4 and IPv6 router ID if the router ID is configured on the loopback interface. Otherwise, the lowest IP address assigned to the loopback interface is chosen as the node SID. Configuring a node SID through policy allows you to choose the loopback address that gets the node SID. If the node SID configuration exists and a policy is defined for node SID selection for the same prefix, then the policy configuration takes precedence.

Designate prefix segment indexes to prefix SIDs, both anycast and node SIDs, that are advertised in IS-IS through policy configuration. Remote routers use the SRGB and the index to derive labels for a specific prefix. After the prefix segment indexes are provisioned, the devices running Junos OS advertise them in one or more of the following IS-IS TLV types by using a new Prefix-SID Sub-TLV (type 3):

  • IP Prefix TLV (type 135)

  • MT IP Prefix TLV (type 235)

  • IPV6 Prefix Reachability TLV (type 236)

  • MT IPV6 Prefix Reachability TLV (type 237)

You can similarly designate prefix segment indexes to prefix SIDs, both anycast and node SIDs, that are advertised in OSPF through policy configuration. Remote routers use this index to consolidate prefixes into respective SRGBs and to derive the segment identifier and forward the traffic destined for a specific prefix.

Anycast Segments

An IGP anycast segment is an IGP prefix segment that identifies a set of routers. An anycast segment enforces forwarding based on the equal-cost multipath-aware shortest-path toward the closest node of the anycast set. Within an anycast group, all the routers advertise the same prefix with the same SID value, which facilitates load balancing.

Prefix SID redistribution into OSPF through policy configuration

Related Documentation

  • Configuring Anycast and Prefix segments in SPRING for IS-IS Protocol
  • Configuring Segment Routing Global Blocks Label Ranges in SPRING for IS-IS Protocol
  • Example: Configuring Segment Routing Global Blocks in SPRING for IS-IS to Increase Network Speed

Change History Table

Feature support is determined by the platform and release you are using. Use Feature Explorer to determine if a feature is supported on your platform.

Release
Description
25.4R1
Starting in Junos OS and Junos OS Evolved Release 25.4R1, you can redistribute Segment Routing (SR) prefix-Segment Identifiers (SIDs) across OSPF IGP instances using route policy without explicitly specifying a prefix-segment index.