6
Label retention modes include liberal and conservative.
{
In liberal mode, an LSR keeps any received label binding regardless of whether the binding is
from the next hop for the FEC or not. This allows for quicker adaptation to route changes but will
waste label resources because LSRs keep extra labels.
{
In conservative mode, an LSR keeps only label bindings that are from the next hops for the FECs.
This allows LSRs to maintain fewer labels but makes LSRs slower in adapting to route changes.
NOTE:
The switch supports only the liberal mode.
MPLS forwarding
LFIB
An LFIB comprises the following table entries.
•
Next Hop Label Forwarding Entry (NHLFE): Describes the label operation to be performed. It is
used to forward MPLS packets.
•
FEC to NHLFE (FTN) map: FTN maps each FEC to a set of NHLFEs at the ingress LSR. The FTN map
is used for forwarding unlabeled packets that need MPLS forwarding. When an LSR receives an
unlabeled packet, it looks for the corresponding FIB entry. If the Token value of the FIB entry is not
Invalid, the packet needs to be forwarded through MPLS. The LSR then looks for the corresponding
NHLFE entry according to the Token value to determine the label operation to be performed.
•
Incoming Label Map (ILM): ILM maps each incoming label to a set of NHLFEs. It is used to forward
labeled packets. When an LSR receives a labeled packet, it looks for the corresponding ILM entry.
If the Token value of the ILM entry is not null, the LSR looks for the corresponding NHLFE entry to
determine the label operation to be performed.
NOTE:
FTN and ILM are associated with NHLFE through Token.
