ECMP support for LDP

This feature performs load balancing for LDP-based LSPs by supporting multiple outgoing next-hops for a specific IP prefix on ingress and transit LSRs.

An LSR that has multiple equal cost paths to a specific IP prefix can receive an LDP label mapping for this prefix from each downstream next-hop peer. The LDP implementation uses the liberal label retention mode to retain all the labels for an IP prefix received from multiple next-hop peers.

Without ECMP support, only one of these next-hop peers is selected and installed in the forwarding plane. The the next-hop peer selection algorithm looks up the route information obtained from the routing table manager (RTM) for this prefix and finds the first valid LDP next-hop peer (for example, the first neighbor in the RTM entry from which a label mapping was received). If the outgoing label to the installed next-hop is no longer valid (for example, if the session to the peer is lost or the peer withdraws the label), a new valid LDP next-hop peer is selected from the existing next-hop peers, and LDP reprograms the forwarding plane to use the label sent by this peer.

With ECMP support, all the valid LDP next-hop peers (peers that sent a label mapping for a specific IP prefix) are installed in the forwarding plane. In ingress LER and transit LSR, an ingress label is mapped to the next-hops that are in the RTM and from which a valid mapping label has been received. The forwarding plane then uses an internal hashing algorithm to determine how traffic will be distributed amongst these multiple next-hops, assigning each flow to a specific next-hop.

For more information about hash algorithms at LER and transit LSR, see "LAG hashing" in the 7210 SAS-D, Dxp, K 2F1C2T, K 2F6C4T, K 3SFP+ 8C Interface Configuration Guide.