3Com Switch 8800 Configuration Guide
Chapter 35 MPLS Architecture
35-6
35.3.3 Establishing LSP
Actually, the establishment of LSP refers to the process of binding FEC with the label,
and then advertising this binding to the adjacent LSR on LSP. This process is
implemented through LDP, which regulates the message in interactive processing and
message structure between LSRs as well as routing mode.
I. LDP working process
Through sending Hello message periodically, an LSR finds its neighbor and then
establish LDP session with the newly discovered adjacent LSR. By LDP session, the
adjacent LSRs advertise such information as label switching mode, label space,
session keepalive timer value to each other. LDP session is a TCP connection, which
needs to be maintained through LDP message. In case there is not any other LDP
message during the time period specified by the session keepalive timer value, and
then it is necessary to send session keepalive message to maintain the existence of
LDP session. Figure 35-4 illustrates the diagram of LDP label distribution.
MPLS edge router (LER)
LSP1
MPLS core router (LSR)
Ingress
Egress
LDP session
LSP2
Label request message
Label map message
A
B
C
D
E
F
G
H
MPLS edge router (LER)
LSP1
MPLS core router (LSR)
Ingress
Egress
LDP session
LSP2
Label request message
Label map message
A
B
C
D
E
F
G
H
Figure 35-4
Label distribution process
For the label distribution mentioned previously, there are two modes: DoD and DU. The
main difference between these two modes is that the label mapping messages are
distributed actively or passively.
In DoD mode, the label is distributed in this way: the upstream LSR sends label request
message (containing FEC descriptive information) to the downstream LSR, and the
downstream LSR distributes label for this FEC, and then it sends the bound label back
to the upstream LSR through label map message. The time when the downstream LSR
feeds back the label map message depends on whether this LSR uses independent
label control mode or sequential label control mode. When the sequential label control
mode is used by the downstream LSR, the label map message is sent back to its
upstream LSR if only it has received the label map message from its downstream LSR.
And when the independent label control mode is used by the downstream LSR, then it
will send label map message to its upstream LSR immediately, no matter whether it has