Fundamental Operation Of Ldp - 3Com MSR 50 Series Configuration Manual

Fundamental Operation
of LDP
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.
LDP goes through four phases in operation: discovery, session establishment and
maintenance, LSP establishment and maintenance, and session termination.
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
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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.
At the end of the discovery phase, Hello adjacency is established between LSRs,
and LDP is ready to initiate session establishment.
Session establishment and maintenance
In this phase, LSRs pass through two steps to establish sessions between them:
1 Establishing transport layer connections (that is, TCP connections) between them.
2 Initializing sessions and negotiating session parameters such as the LDP version,
label distribution mode, timers, and label spaces.
LDP Overview 1323