Cisco Crosswork Planning Design 7.2.x User Guide

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Cisco Crosswork Planning Design 7.2.x User Guide

SR LSP routing rules

Updated: August 14, 2026

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Provides rules and behaviors for how SR LSPs route demands using segment lists, path selection, and node reachability to achieve efficient, flexible traffic routing.

These rules determine how SR LSPs route demands through a network, assuming the common LSP mechanisms such as autoroute and forwarding adjacencies have determined that a demand will enter an SR LSP. If the destination node or segment list hops are not reachable, the SR LSP is not established.

Note

The process of creating final segment list hops can be automated. For more information, refer to Simulate Traffic Flow from Source to Destination Using Demands.

  • If the SR LSP does not have a segment list, the demand is routed via IGP to the destination node.

    If the SR LSP has a segment list, the demand is routed using the segment list hops within the segment list in the sequential order in which they appear.

  • If it is using a node segment list hop, the demand takes the shortest path to that node.

    If it is using an interface segment list hop, the demand takes the shortest path of the source node of that interface, and then routes to that interface’s destination node.

    If it is using an anycast group segment list hop, the demand routes through the node with the shortest path. In case of a tie between multiple nodes in an anycast group, the SR LSP uses ECMP to route.

  • If you select both a node and an anycast group containing that node when creating the segment list hop, the node segment has precedence.

  • If the SR LSP contains LSP paths, the demand is routed on the LSP path with the lowest path option for which the demand can be routed from source to destination. Demands for SR LSP paths route in the same manner as described above for SR LSPs.

  • If the SR LSP contains multiple SR candidate paths, the demand is routed on the LSP path with the highest preference option for which the demand can be routed from source to destination. Demands for SR LSP paths route in the same manner as described above for SR LSPs.

  • If an SR LSP path is configured as 'Dynamic', the LSP path is simulated based on the specified 'Metric type'. This is applicable for IGP, TE, and Delay metric types. As ECMP is not allowed along the path, interfaces with the lowest IP address are preferred. If there is a segment list configured, the LSP path is not dynamically allocated.

  • If the demand cannot be routed on the segments defined in the SR LSP or its LSP paths (for example, because of a failure in a node segment list hop), the demand is routed to the destination using the IGP shortest path.

  • If a hop (nodes, interfaces, LSPs, or anycast groups) contains a segment ID (SID), the segment list hop inherits that SID.

Inter-AS LSP routing

An inter-AS LSP routing is a network routing mechanism that enables SR LSPs to traverse multiple ASes.

To route SR LSPs with source and destination nodes in different ASes, these hop configurations must exist:

  • A node segment list hop on the border of the AS that the LSP is exiting. This node is the exit node.

  • A node segment list hop on the first node traversed in another AS. This node is the entry node. If there are multiple peering interfaces, their metrics determine which one the LSP uses.

  • An interface segment list hop that connects the exit node of one AS to an entry node in another AS.

To use demands with inter-AS SR LSPs, these two conditions must be met:

  • The demand must be private to the SR LSP. For information on creating private LSPs, refer to Configure MPLS Routing.

  • The demand must have the same source and destination as the SR LSP.