An LSR can use different modes when distributing labels to other LSRs. This section covers three distinct modes, as follows:
Label distribution mode
Label retention mode
LSP control mode
Each mode has its own characteristics. This section explains the advantages of each.
The MPLS architecture has two modes to distribute label bindings:
Downstream-on-Demand (DoD) label distribution mode
Unsolicited Downstream (UD) label distribution mode
In the DoD mode, each LSR requests its next-hop (that is, downstream) LSR on an LSP, a label binding for that FEC. Each LSR receives one binding per FEC only from its downstream LSR on that FEC. The downstream LSR is the next-hop router indicated by the IP routing table. ...