H3C Technologies H3C S12500-X Series Switches - Manual (Page 15)

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MPLS forwarding
Figure 5 MPLS forwarding
As shown in Figure 5 , a packet is forwarded over the MPLS network in the following steps:
1. Router B (the ingress LSR) receives a packet with no label. It identifies the FIB entry that matches the
destination address of the packet, pushes the outgoing label (40 in this example) to the packet,
and forwards the labeled packet out of the interface VLAN-interface 20 to the next hop LSR Router
C.
2. When receiving the labeled packet, Router C identifies the LFIB entry that has an incoming label of
40, uses the outgoing label 50 of the entry to replace label 40 in the packet, and forwards the
labeled packet out of the outgoing interface VLAN-interface 30 to the next hop LSR Router D.
3. When receiving the labeled packet, Router D (the egress) identifies the LFIB entry that has an
incoming label of 50, removes the label from the packet, and forwards the packet out of the
outgoing interface VLAN-interface 40 to the next hop LSR Router E. If the LFIB entry records no
outgoing interface or next hop information, Router D identifies the FIB entry by the IP header and
then forwards the packet according to the FIB entry.
PHP
An egress node must perform two forwarding table lookups to forward a packet: two LFIB lookups (if the
packet has more than one label), or one LFIB lookup and one FIB lookup (if the packet has only one
label).
The penultimate hop popping (PHP) feature can pop the label at the penultimate node, so the egress
node only performs one table lookup.
A PHP-capable egress node sends the penultimate node an implicit null label of 3. This label never
appears in the label stack of packets. If an incoming packet matches an LFIB entry comprising the implicit
null label, the penultimate node pops the top label of the packet and forwards the packet to the egress
LSR. The egress LSR directly forwards the packet.