Fundamental Operation Of Ldp - H3C S9500 Series Operation Manual

Operation Manual – MPLS Basics
H3C S9500 Series Routing Switches
In independent mode, an LSR immediately responds to its upstream LSR with
label binding information no matter whether it receives that of its downstream LSR
or not.
Usually, an upstream LSR selects its downstream LSR based on the information in its
routing table. In
LSP 2 works in independent mode.
II. DU mode
In DU mode, a downstream LSR advertises label binding information to its upstream
LSR unsolicitedly after the LDP session is established, while the upstream LSR keeps
the label binding information and processes the information based on its routing table
information.

1.3.3 Fundamental Operation of LDP

LDP goes through four phases in operation: Discovery,
maintenance,
I. Discovery
In this phase, an LSR who wants to establish a session sends Hello messages to its
neighboring LSRs periodically, announcing its presence. This way, LSRs can
automatically find their peers without manual configuration.
LDP provides two discovery mechanisms:
Basic discovery mechanism
The basic discovery mechanism is used to discover local LDP peers, that is, LSRs
directly connected at link layer, and to further establish local LDP sessions.
Using this mechanism, an LSR periodically sends LDP link Hellos as UDP packets out
an interface to the multicast address known as "all routers on this subnet". An LDP link
Hello message carries information about the LDP identifier of a given interface and
some other information. Receipt of an LDP link Hello message on an interface indicates
that a potential LDP peer is connected to the interface at link layer.
Extended discovery mechanism
The extended discovery mechanism is used to discover remote LDP peers, that is,
LSRs not directly connected at link layer, and to further establish remote LDP sessions.
Using this mechanism, an LSR periodically sends LDP targeted Hellos as UDP packets
to a given IP address.
An LDP targeted Hello message carries information about the LDP identifier of a given
LSR and some other information. Receipt of an LDP targeted Hello message on an
LSR indicates that a potential LDP peer is connected to the LSR at network layer.
Figure 1-7, all LSRs on LSP 1 work in ordered mode, while LSR F on
LSP establishment and
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Chapter 1 MPLS Basics Configuration
Session establishment and
maintenance, and Session termination.