61Label distribution controlLDP controls label distribution in one of the following ways:• Independent label distribution—Distributes a FEC-label mapping to an upstream LSR at any time.An LSR may distribute a mapping for a FEC to its upstream LSR before it receives a label mappingfor that FEC from its downstream LSR. As shown in Figure 17, in DU mode, each LSR distributes alabel mapping for a FEC to its upstream LSR whenever it is ready to label-switch the FEC, withoutwaiting for a label mapping for the FEC from its downstream LSR. In DoD mode, an LSR distributesa label mapping for a FEC to its upstream LSR after it receives a label request for the FEC, withoutwaiting for a label mapping for the FEC from its downstream LSR.Figure 17 Independent label distribution control mode• Ordered label distribution—Distributes a label mapping for a FEC to its upstream LSR only after itreceives a label mapping for that FEC from its downstream LSR unless the local node is the egressnode of the FEC. As shown in Figure 16, in DU mode, an LSR distributes a label mapping for a FECto its upstream LSR only if it receives a label mapping for the FEC from its downstream LSR. In DoDmode, when an LSR (Transit) receives a label request for a FEC from its upstream LSR (Ingress), itcontinues to send a label request for the FEC to its downstream LSR (Egress). After the transit LSRreceives a label mapping for the FEC from the egress LSR, it distributes a label mapping for the FECto the ingress.Label retention modeThe label retention mode specifies whether an LSR maintains a label mapping for a FEC learned from aneighbor that is not its next hop.• Liberal label retention—Retains a received label mapping for a FEC regardless of whether theadvertising LSR is the next hop of the FEC. This mechanism allows for quicker adaptation totopology changes, but it wastes system resources because LDP has to keep useless labels. Thedevice only supports liberal label retention.• Conservative label retention—Retains a received label mapping for a FEC only when theadvertising LSR is the next hop of the FEC. This mechanism saves label resources, but it cannotquickly adapt to topology changes.
