There are two types of MPLS label distribution control methods:
1. Independent label distribution control (Independent): The LSR can advertise label mapping to the LSR connected to it at any time. This approach may cause the label to be issued upstream before the downstream label is received.
2. Ordered label control mode (Ordered): For the label mapping of a certain FEC on the LSR, only when the LSR already has the label mapping message of the FEC next hop or the LSR is the exit node of the FEC, the LSR can The label mapping of this FEC can be sent upstream.
Label Retention Mode
The label retention mode refers to how the LSR processes the FEC binding of the label received but not used temporarily.
There are also two label retention methods:
Free label retention mode (Liberal): The label mapping received from the neighbor LSR is reserved regardless of whether the neighbor LSR is its next hop or not.
Conservative label retention mode (Conservative): The label mapping received from the neighbor LSR is reserved only when the neighbor LSR is its next hop.
Using the free label retention mode, LSR can quickly adapt to routing changes; while using the conservative label retention mode, LSR can allocate and save a smaller number of labels.
The conservative label retention method is usually used together with the DoD method for LSR with limited label space.
Several basic concepts in label switching
NHLFE (Next Hop Label Forwarding Entry): Next Hop Label Forwarding Entry. Used to describe the operations performed on the tag, including push operations and swap operations.
FTN (FEC to NHLFE map): The process of mapping forwarding equivalence FEC to NHLFE at the Ingress node.
ILM (Incoming Label Map): Incoming Label Map. For the received label packet, the LSR maps the label to the NHLFE process, including Null operation and Pop operation.