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Preserve Next Hop Hierarchy

In traditional compressed hierarchy, service labels are merged into a single transport nexthops (FNH), which can lead to nexthop explosion in the PFE. Traditional compressed hierarchy can also cause issues like ECMP truncation and loss of hierarchical weights.

Preserve Next Hop Hierarchy (also known as Expanded Hierarchy) keeps each service label in its own chain composite, preserving the original hierarchy instead of merging everything into a single nexthop structure.

Benefits of Preserve Next Hop Hierarchy

This feature provides the following benefits:

  • Reduction of Forwarding Nexthop Resources: Provides significant reduction in forwarding Nexthop resources/count.
  • BGP PIC at All Levels: Supports BGP PIC across all hierarchy levels (transport, service, composite chains), ensuring fast failover and local convergence for BGP paths.
  • Preservation of Hierarchical Weights: Maintains weight and bandwidth distribution for ECMP and hierarchical routing scenarios.
  • Avoid ECMP Truncation: Ensures all ECMP legs are installed in PFE, improving load balancing and resiliency.

Services such as Layer 2 services (Layer 2 circuit, Layer 2 VPN), Layer 3 VPN (IPv4 and IPv6), VPLS, IPIP tunnels, Internet services (IPv4, IPv6, 6PE), EVPN and PRPD support Preserve Next Hop Hierarchy.

Transport protocols such as BGP-Labeled Unicast, BGP Classful Transport (BGP-CT), RSVP-TE, SR-MPLS, SR-FlexAlgo, SR-TE, SRv6 TE, MPLS-over-UDP support Preserve Next Hop Hierarchy with Transport Class infrastructure.

See Feature Explorer tool for the list of platforms that support Preserve Next Hop Hierarchy.

Configure Preserve Next Hop Hierarchy

To configure the Preserve Next Hop Hierarchy feature, include the preserve-nexthop-hierarchy statement at the [edit routing-options resolution] hierarchy level. This ensures that all related routing processes respect the established next hop hierarchy. This facilitates efficient route resolution and quicker adaptation in case of path failures, contributing to enhanced network performance.

The Preserve Next Hop Hierarchy feature benefits networks that require high availability and seamless integration with advanced technologies like Segment Routing with MPLS and Segment Routing with IPv6. They offer improved traffic distribution, redundancy, and consistent routing behavior across complex network architectures.

Note: When you enable or disable the preserve-nexthop-hierarchy statement either at global or at protocol levels, the nexthop pointed by the affected routes is recomputed and downloaded to PFE and kernel. This results in higher CPU spikes in the device in a higher scale environment leading to packet loss until the routes are reprogrammed in the PFE.