LDP LSP Establishment

Process of Establishing an LSP

The LSP establishment is the process of binding a FEC with a label and advertising the binding to LSRs. The procedure for establishing an LSP in DU label distribution mode and ordered label control mode is as follows:

1. By default, if an LER finds a new host route in the routing table and the destination IP address in the host route is mapped to no existing FEC, the LER creates a FEC for the destination IP address.
2. If the egress has available labels, it distributes labels for FECs and proactively sends a Label Mapping message to an upstream transit LSR. The Label Mapping message contains distributed labels and bound FECs.
3. After receiving the Label Mapping message, a transit LSR adds the mapping to its label forwarding table and then proactively sends a Label Mapping message of the specified FEC to the ingress.
4. After receiving the Label Mapping message, the ingress also adds the mapping to its label forwarding table. An LSP is established, and the packets classified as the FEC can be forwarded based on the label.

LSP Load Balancing

Equal-cost LDP LSPs for the same FEC can be established on the ingress or a transit node to balance traffic. Equal-cost LDP LSPs are established using equal-cost routes, including IGP routes and static routes.