H3C Technologies H3C SR8800 - Manual (Page 18)

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corresponding NHLFE entry according to 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 when
forwarding 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 will look for the corresponding
NHLFE entry to determine the label operation to be performed.
NOTE:
FTN and ILM are associated with NHLFE through Token.
MPLS data forwarding
Figure 7 MPLS forwarding process diagram
As shown in Figure 7 , in an MPLS domain, a packet is forwarded in the following procedure:
1. The ingress (Router B) receives a packet carrying no label. Router B determines the FEC of the
packet according to the destination address, and searches the FIB table for the Token value. As the
Token value is not Invalid, Router B looks for the corresponding NHLFE entry of the Token value.
According to the NHLFE entry, Router B pushes label 40 to the packet, and then forwards the
labeled packet to the next hop LSR (Router C) through the outgoing interface (GE3/0/2).
2. Upon receiving the labeled packet, Router C looks for the ILM entry according to the label (40) to
get the Token value. As the Token value is not null, Router C looks for the corresponding NHLFE
entry of 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 (GE3/0/2).
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. As the Token is null, 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.
IP
:40
.1.1.2
IP:40.
1.1.2