ECMP and weighted ECMP for services using RSVP and SR-TE LSPs

ECMP over MPLS LSPs refers to spraying packets across multiple named RSVP and SR-TE LSPs within the same ECMP set. The ECMP-like spraying consists of hashing the relevant fields in the header of a labeled packet and selecting the next-hop tunnel based on the modulo operation of the output of the hash and the number of ECMP tunnels. Only LSPs with the same lowest LSP metric can be part of the ECMP set.

In weighted ECMP, the load-balancing weight of the LSP is normalized by the system and then used to bias the amount of traffic forwarded over each LSP. Weighted ECMP is supported where the service resolves directly to an ECMP set of RSVP or SR-TE LSPs with a configured load balancing weight, or where it resolves to a BGP tunnel and then uses an ECMP set of RSVP or SR-TE LSPs with a configured load balancing weight. Use the following commands to configure the weight for an LSP or LSP template.

configure router mpls lsp load-balancing-weight
configure router mpls lsp-template load-balancing-weight

If one or more LSPs in the ECMP set do not have load-balancing-weight configured, and the ECMP is set to a specific next hop, regular ECMP spraying is used.

Weighted ECMP is supported for VPRN Layer 3 services. See the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: IES and VPRN for more information.

Weighted ECMP is supported for the following Layer 2 services over RSVP-TE (including LDP over RSVP) and SR-TE tunnels:

  • Epipe VLLs

  • Ipipe VLLs

  • LDP VPLS

  • BGP-AD VPLS with provisioned SDPs

Class Based Forwarding (CBF) and weighted ECMP are mutually exclusive for VLL and VPLS services.

For services that use an explicitly configured SDP, use the following command to configure weighted ECMP under the SDP used by the service.

configure service sdp weighted-ecmp

By default, weighted ECMP is disabled.

For VLL and VPLS services that use a provisioned SDP on which weighted ECMP is configured, a path is selected based on the configured hash. Paths are then load-balanced across LSPs within an SDP according to the normalized LSP weights. Additional fields may be taken into account for VPLS services based on the commands configured in the following context.

configure service vpls load-balancing
configure service epipe load-balancing

Weighted ECMP is also supported for EVPN services. See the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide for more information.