3com S7906E 설치 설명서
1-13
upstream LSR. However, the time when the downstream LSR sends label binding information depends
on the label distribution control mode that it uses:
on the label distribution control mode that it uses:
z
In ordered mode, a downstream LSR sends label binding information only after it receives that of its
downstream LSR.
downstream LSR.
z
In independent mode, a downstream LSR sends label binding information immediately after it
receives a label request message, no matter whether it has received the label binding information
of its downstream LSR or not.
receives a label request message, no matter whether it has received the label binding information
of its downstream LSR or not.
Usually, an upstream LSR selects its downstream LSR based on the information in its routing table. In
, all LSRs along LSP 1 work in ordered mode, while LSR F along LSP 2 works in independent
mode.
DU mode
In DU mode, an LSR advertises label binding information to all its neighboring LSRs unsolicitedly after
the LDP sessions are established. An LSR receiving the label binding information determines how to
process the label binding information based on its label retention mode and routing table information.
the LDP sessions are established. An LSR receiving the label binding information determines how to
process the label binding information based on its label retention mode and routing table information.
Fundamental Operation of LDP
LDP goes through four phases in operation: discovery, session establishment and maintenance, LSP
establishment and maintenance, and session termination.
establishment and maintenance, and session termination.
Discovery
In this phase, an LSR wanting 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.
periodically, announcing its presence. This way, LSRs can automatically find their peers without manual
configuration.
LDP provides two discovery mechanisms:
z
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.
link layer, and to further establish local LDP sessions.
Using this mechanism, an LSR periodically sends LDP link Hello messages in the form of UDP packets
out its interfaces 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.
out its interfaces 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.
z
Extended discovery mechanism
The extended discovery mechanism is used to discover remote LDP peers, that is, LSRs that are not
directly connected at link layer, and to further establish remote LDP sessions.
directly connected at link layer, and to further establish remote LDP sessions.
Using this mechanism, an LSR periodically sends LDP targeted Hello messages in the form of UDP
packets to a given IP address.
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.
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.
initiate session establishment.