HP HSR6800 Configuration Manual page 14

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.
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Incoming Label Map—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 7 MPLS forwarding process diagram
In an MPLS domain, a packet is forwarded in the following procedure:
1. 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 1/0/2).
2. Upon receiving the labeled packet, Router C looks for the ILM entry that contains the label 40 to
get the Token value. Because the Token value is not blank, Router C looks for the NHLFE entry
containing the Token value. According to the NHLFE entry, Router C swaps the original label
with label 50, and then forwards the labeled packet to the next hop LSR (Router D) through the
outgoing interface (GigabitEthernet 1/0/2).
3. Upon receiving the labeled packet, Router D (the egress) looks for the ILM entry according to
the label (50) to get the Token value. Because the Token is blank, Router D removes the label
from the packet. If the ILM entry records the outgoing interface, Router D forwards the packet
through the outgoing interface. If no outgoing interface is recorded, router D forwards the
packet according to the IP header of the packet.
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