R211x-HP Flexfabric 11900 MPLS Configuration Guide

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.

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

packet matches an LFIB entry comprising the explicit null label, the penultimate hop replaces the value of

the top label with value 0, and forwards the packet to the egress node. The egress node gets the TC

information, pops the label of the packet, and forwards the packet.

Protocols and standards

• RFC 3031, Multiprotocol Label Switching Architecture

• RFC 3032, MPLS Label Stack Encoding

• RFC 5462, Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic

Class" Field

MPLS configuration task list

Tasks at a

lance

(Required.) Enabling MPLS

(Optional.) Configuring MPLS MTU

(Optional.) Specifying the label type advertised by the egress

(Optional.) Configuring TTL propagation

(Optional.) Enabling sending of MPLS TTL-expired messages

(Optional.) Enabling SNMP notifications for MPLS

Enabling MPLS

You must enable MPLS on all interfaces related to MPLS forwarding.

Before you enable MPLS, complete the following tasks: