Label Advertisement Modes

LDP provides two modes for advertising labels:

Downstream-on-demand (DoD)
Downstream unsolicited (DU)

Using DoD mode, label bindings are only distributed in response to explicit requests. A typical LSP establishment flow begins when the ingress LER originates a label request message to request a label binding for a particular FEC (for a particular IP address prefix or IP host address). The label request message follows the normal routed path to the FEC. The egress LER responds with a label mapping message that includes a label binding for the FEC. The label mapping message then follows the routed path back to the ingress LSR, and a label binding is provided by each LSR along the path. LSP establishment is complete when the ingress LER receives the label mapping message.

Conversely, using DU mode, an LSR may distribute label bindings to LSRs that have not specifically requested them. These bindings are distributed using the label mapping message, as in downstream-on-demand mode. From an LDP message perspective, the primary difference using DU mode is the lack of a preceding label request message.

Architecturally, the difference is more significant, because the DU mode is often associated with a topology-driven strategy, where labels are routinely assigned to entries as they are inserted into the routing database. In either case, an LSR only uses a label binding to switch traffic if the binding was received from the current next hop for the associated FEC.

Both label advertisement modes can be concurrently deployed in the same network. However, for a given adjacency, the two LSRs must agree on the discipline. Negotiation procedures specify that DU mode be used when a conflict exists when using Ethernet links. Label request messages can still be used when MPLS (Multiprotocol Label Switching) is operating in unsolicited mode.