LDP provides two modes for label retention:
Using conservative label retention mode, an LSR retains only the label-to-FEC mappings that it currently needs (mappings received from the current next hop for the FEC).
Using liberal retention mode, LSRs keep all the mappings that have been advertised to them. The trade-off is memory resources saved by conservative mode versus the potential of quicker response to routing changes made possible by liberal retention (for example, when the label binding for a new next hop is already resident in memory).
The Extreme MPLS implementation supports liberal label retention only.