7
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: 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.
{
Extended discovery mechanism
—Discovers indirectly connected LDP peers and establishes targeted
hello adjacencies. An LSR periodically sends LDP Hello messages to a given IP address so that the LSR with
the IP address can discover the LDP peer.