HP 12500 Series Configuration Manual page 15

MPLS data forwarding
Figure 7 MPLS forwarding process diagram
As shown in
Router B (the ingress LSR) receives a packet carrying no label. It determines the FEC of the packet
1.
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 (GE3/0/2).
Upon receiving the labeled packet, Router C looks for the ILM entry that contains the label 40 to
2.
get the Token value. Because the Token value is not empty, Router C looks for the corresponding
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 (GE3/0/2).
Upon receiving the labeled packet, Router D (the egress) looks for the ILM entry according to the
3.
label (50) to get the Token value. Because the Token is empty, 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.
PHP
In an MPLS network, when an egress node receives a labeled packet, it looks up the LFIB, pops the label
of the packet, and then performs the next level label forwarding or performs IP forwarding. The egress
node needs to do two forwarding table lookups to forward a packet: looks up the LFIB twice, or looks up
the LFIB once and the FIB once.
The penultimate hop popping (PHP) feature can pop the label at the penultimate node to relieve the
egress of the label operation burden.
PHP is configured on the egress node. The label assigned by a PHP-capable egress node to the
penultimate hop can be one of the following:
Figure 7, in an MPLS domain, a packet is forwarded in the following procedure:
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