4000 Router Series MPLS Configuration Guide

ManualsBrandsHP ManualsNetwork HardwareHP MSR3024 TAA-compliant AC Router

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 Ethernet 1/2 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 Ethernet 1/2 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 Ethernet 1/2 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.

Sometimes, the egress node must use the TC field in the label to perform QoS. To keep the TC information,

you can configure the egress node to send the penultimate node an explicit null label of 0. If an incoming

Router A

Router B

Ingress

Router C

Router D

Egress

Router E

Router F

IP:10.1.1.1

MPLS network

IP:10.1.1.140 IP:10.1.1.1 50 IP:10.1.1.1

Eth1/1 Eth1/2 Eth1/1 Eth1/2 Eth1/1 Eth1/2

FIB table

LFIB table

NexthopDest Out intOut label

10.1.0.0

Eth1/240

Router C

Nexthop

Router D

Out int

Eth1/2

Out label

Oper

Swap

In label