This section describes the MPLS forwarding information.
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 must 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.
FTN and ILM are associated with NHLFE through Token.
MPLS data forwarding
Figure 18 MPLS forwarding process diagram
In an MPLS domain, a packet is forwarded in the following procedure:
Router B (the ingress LSR) receives a packet carrying no label. It determines the FEC of the packet
according to the destination address, and searches the FIB table for the Token value. Because the
Token value is not Invalid, Router B looks for the corresponding NHLFE entry that contains the
Token value. According to the NHLFE entry, Router B pushes label 40 to the packet, and forwards
the labeled packet to the next hop LSR (Router C) through the outgoing interface (GigabitEthernet