Label distribution and control

LDP advertises label-FEC mappings in one of the following ways:

• Downstream Unsolicited (DU) mode—Distributes FEC-label mappings to the upstream LSR, without waiting for label requests. The device supports only the DU mode.

• Downstream on Demand (DoD) mode—Sends a label request for an FEC to the downstream LSR. After receiving the label request, the downstream LSR distributes the FEC-label mapping for that FEC to the upstream LSR.

	NOTE: A pair of upstream and downstream LSRs must use the same label advertisement mode. Otherwise, the LSP cannot be established.

LDP controls label distribution in one of the following ways:

• Independent label distribution—Distributes an FEC-label mapping to an upstream LSR at any time. An LSR might distribute a mapping for an FEC to its upstream LSR before it receives a label mapping for that FEC from its downstream LSR. As shown in Figure 11, in DU mode, each LSR distributes a label mapping for an FEC to its upstream LSR whenever it is ready to label-switch the FEC, without waiting for a label mapping for the FEC from its downstream LSR. In DoD mode, an LSR distributes a label mapping for an FEC to its upstream LSR after it receives a label request for the FEC, without waiting for a label mapping for the FEC from its downstream LSR.

  Figure 11: Independent label distribution control mode

• Ordered label distribution—Distributes a label mapping for an FEC to its upstream LSR only after it receives a label mapping for that FEC from its downstream LSR unless the local node is the egress node of the FEC. As shown in Figure 10, in DU mode, an LSR distributes a label mapping for an FEC to its upstream LSR only if it receives a label mapping for the FEC from its downstream LSR. In DoD mode, when an LSR (Transit) receives a label request for an FEC from its upstream LSR (Ingress), it continues to send a label request for the FEC to its downstream LSR (Egress). After the transit LSR receives a label mapping for the FEC from the egress LSR, it distributes a label mapping for the FEC to the ingress.

The label retention mode specifies whether an LSR maintains a label mapping for an FEC learned from a neighbor that is not its next hop.

• Liberal label retention—Retains a received label mapping for an FEC regardless of whether the advertising LSR is the next hop of the FEC. This mechanism allows for quicker adaptation to topology changes, but it wastes system resources because LDP has to keep useless labels. The device only supports liberal label retention.

• Conservative label retention—Retains a received label mapping for an FEC only when the advertising LSR is the next hop of the FEC. This mechanism saves label resources, but it cannot quickly adapt to topology changes.