ECMP programming for LDP transit LSP creates a set of ECMP paths on the forwarding plane at any transit router. LDP LSPs transit traffic is load balanced using programmed ECMP. The number of ECMP paths that are used depends on the number of eligible paths that are available, and the maximum number of LDP ECMP paths that you configure.
The Routing Information Base (RIB) Manager controls the number of available paths sent to LDP which is limited by IP load sharing. LDP also enables its own load sharing limit. The lesser of the two load sharing limits form the maximum number of ECMP paths that can be programmed on forwarding plane.
When new ECMP paths are added, or existing paths are deleted from a set of eligible ECMP paths, MPLS forwarding decides when these changes lead to a different set of paths to be used for LDP LSP, ingress tunnel, or transit LSP.
When a different set of paths are used, updates are sent to the forwarding plane. MPLS only sends an update to the forwarding plane when there is a change to the set of programmed paths. MPLS always sends the complete set of ECMP paths to the forwarding plane. When you change the load sharing configuration, updates are also sent to the forwarding plane. FEC updates are only generated when the new load sharing value is different from the set of ECMP paths programmed in the forwarding plane.
Note
LDP ECMP is not supported at the ingress router.The ingress LDP LSP can be different from the transit LSP for the same FEC.
MPLS Operations, Administration, and Maintenance (OAM) support for traceroute at any transit router returns the list of labels used at that transit router. However, traceroute is not able to exercise all ECMP paths. The forwarding plane selects one ECMP path to forward OAM packets. All traversed labels that were returned at each transit router are displayed at the Extreme router originating the traceroute.