Label Distribution Protocol (LDP) is used to distribute labels in non-traffic-engineered applications. LDP allows routers to establish LSPs through a network by mapping network-layer routing information directly to data link LSPs.
An LSP is defined by the set of labels from the ingress LER to the egress LER. LDP associates a Forwarding Equivalence Class (FEC) with each LSP it creates. A FEC is a collection of common actions associated with a class of packets. When an ingress LER assigns a label to a FEC, it must allow other LSRs in the path know about the label. LDP helps to establish the LSP by providing a set of procedures that LSRs can use to distribute labels.
The FEC associated with an LSP specifies which packets are mapped to that LSP. LSPs are extended through a network by each LSR, where each LSR splices incoming labels for the FEC to the outgoing label assigned to the next hop for the FEC.
LDP allows an LSR to request a label from a downstream LSR so it can bind the label to a specific FEC. The downstream LSR responds to the request from the upstream LSR by sending the requested label.
LSRs can distribute a FEC label binding in response to an explicit request from another LSR. This is known as Downstream On Demand (DOD) label distribution. LSRs can also distribute label bindings to LSRs that have not explicitly requested them. This is called Downstream Unsolicited (DU). For LDP on the 7705 SAR, Downstream Unsolicited (DU) mode is implemented.
This section contains the following topics: