R3303-HP HSR6800 Routers MPLS Configuration Guide

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node needs to do two forwarding table lookups to forward a packet: looking up the LFIB twice or looking
up the LFIB and the FIB once each.
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 two listed:
• IPv4 explicit null label 0—The egress assigns an IPv4 explicit null label to a FEC and advertises the
FEC-label binding to the upstream LSR. When forwarding an MPLS packet, the upstream LSR
replaces the label at the stack top with the explicit null label and then sends the packet to the egress.
When the egress receives the packet, which carries a label of 0, it does not look up for the LFIB entry
but pops the label stack directly and performs IPv4 forwarding.
• Implicit null label 3—This label never appears in the label stack. An LSR directly performs a pop
operation to the labeled packets that match the implicit null label rather than substituting the implicit
null label for the original label at the stack top. After that, the LSR forwards the packet to the
downstream egress LSR. The egress directly performs the next level forwarding upon receiving the
packet.
LDP
LDP establishes LSPs dynamically. Using LDP, LSRs can map network layer routing information to data link
layer switching paths.
Basic concepts of LDP
• LDP session—LDP sessions are established between LSRs over TCP connections to exchange
messages for label binding, label releasing, and error notification.
• LDP peer—Two LSRs using LDP to exchange FEC-label bindings are LDP peers.
LDP message type
LDP messages include the following types:
• Discovery messages—Declare and maintain the presence of LSRs.
• Session messages—Establish, maintain, and terminate sessions between LDP peers.
• Advertisement messages—Create, alter, and remove FEC-label bindings.
• Notification messages—Provide advisory information and notify errors.
LDP session, advertisement, and notification messages use TCP for reliability. Discovery messages use
UDP for efficiency.
LDP operation
LDP goes through the following phases in operation:
1. Discovery:
Each LSR sends hello messages periodically to notify neighboring LSRs of its presence. In this way,
LSRs can automatically discover their LDP peers. LDP provides the following discovery
mechanisms:
{ Basic discovery mechanism—Discovers directly connected LSRs and establishes link hello
adjacencies with them. An LSR periodically sends LDP link Hello messages to multicast address
224.0.0.2 that identifies all routers on the subnet to advertise its presence.